Gaza City (CNN) -- The Israeli military denied Sunday that it was responsible for anyone killed last week when a mortar hit the courtyard of a U.N. school that was shelter to many Gaza residents.
Officials from the United Nations and the Palestinian government said 16 people were killed and hundreds wounded on Thursday when the school in northern Gaza was struck.
"A single errant Israeli mortar landed in the courtyard in the school," Israel Defense Forces spokesman Lt. Col. Peter Lerner said. "The footage we have shows the courtyard was empty."
Lerner said there was fierce fighting between the IDF and Hamas on Thursday. The militants fired anti-tank missiles from the immediate vicinity of the school and the IDF fired several mortars in that direction.
"We reject the claim that people were killed by the IDF mortar on the school premises," he said, adding there could have been people who were wounded by shrapnel.
The U.N. Relief and Workers Agency called for a full investigation of the school shelling, but didn't say by whom. Agency spokesman Chris Gunness said the agency wanted to investigate the incident itself but could not because of "firing at the school."
"We had notified the Israeli military that this school was being used by UNRWA as a shelter for hundreds of people and we notified the Israeli military that there were people there to evacuate and UNRWA never got the approval to evacuate," Gunness said.
CNN team saw evidence of courtyard casualties
A CNN team that visited the shelter several hours after the mortar attack saw evidence that people were badly wounded, if not killed, at the courtyard.
The team saw blood and strewn possessions concentrated close to the edge of the courtyard along the wall of the building, the area that would have been shady around 3 p.m. when the school was hit.
At other U.N. shelters visited in days before this incident, CNN reporters saw displaced persons sitting at the edges of courtyards to take advantage of the shade.
The IDF released a high-altitude aerial video of the round hitting the school, but it did not have high resolution and it is impossible to tell if anybody was sitting on the courtyard edge. CNN has asked the IDF for a higher resolution version of the video, as well as a version that includes a time stamp.
The team observed a shrapnel field ranging from a few inches above the ground to the top of the main three-story school building, with the blast field extending down the corridor of the main school building.
The courtyard was marked by a single detonation point, which would be consistent with what the IDF video shows.
Security experts CNN consulted said the shallow point of detonation was consistent with a mortar round set to "airburst," meaning it would explode a few feet above the ground to maximize enemy casualties.
The team noticed a blast radius of 30 to 40 meters, which CNN security consultants said would be consistent with the damage caused by a 60 mm or 81 mm mortar round. The IDF's main battle tank, the Merkavas, is fitted with a 60 mm mortar.
Speaking to CNN reporters on Sunday, Israeli government spokesman Mark Regev said "the fog of war" is contributing to confusion about what happened at the shelter.
"That people were killed I don't deny," he said. "The question is whose ordnance was it and in what circumstances did it happen."
Both sides renew violence after temporary cease-fire
The news of the IDF investigation came as Hamas and Israel began firing again at each other, renewing the recent violence that has taken more than 1,000 lives.
A temporary truce in the conflict Saturday had enabled medical supplies to be brought into Gaza, families to emerge from shelters and people to dig out the dead from piles of rubble.
But Sunday, Israel said that because of "incessant rocket fire" out of Gaza, it had restarted its Operation Protective Edge, even as Hamas said it agreed to a 24-hour U.N.-mediated cease-fire.
Israeli Prime Minister Benjamin Netanyahu dismissed the call by Hamas for a cease-fire, saying the group violated its own call for a brief stoppage in violence.
"We hope we can get a sustainable quiet as soon as possible. I think the only path to do that is by adopting the Egyptian initiative" and by addressing two underlying issues, he told CNN's "State of the Union with Candy Crowley." Those issues are peace for Israel through demilitarizing Gaza and social and economic relief for the residents of Gaza, he said.
The White House said President Obama and Netanyahu talked by phone on Sunday about Gaza, with Obama stressing the need for an immediate cease-fire in Gaza that would lead to a permanent end of hostilities based on the November 2012 cease-fire agreement.
"The President reaffirmed the United States' support for Egypt's initiative, as well as regional and international coordination to end hostilities," the White House said.
The U.N. Security Council planned to meet at midnight Sunday (12:00 a.m. ET) to discuss the Middle East situation, including the Palestinian question, said a U.N. spokesperson.
Hamas, the militant group that is in control of the besieged Palestinian territory, has rejected Egyptian officials' request to accept a cease-fire, then enter dialogue to discuss its broader concerns.
Hamas militants began firing rockets again Sunday afternoon, blaming a "lack of commitment" from Israel. The Qassam Brigades said on its websites it launched five rockets at Kiryat Gat. The announcement coincided with an Israel Defense Forces tweet that said it intercepted four rockets and one hit an open area.
Another rocket Sunday hit a house, slightly wounding an Israeli civilian. Mortar shells have also landed in Israel, an Eshkol regional spokesman said.
The Israeli Security Cabinet had agreed to a U.N. request late Saturday to extend a cease-fire that started Saturday morning until midnight Sunday (5 p.m. ET Sunday), on the condition that its military could keep dismantling and destroying Hamas' tunnels, according to senior Israeli officials.
Israel said it resumed its offensive because it was still being attacked.
"Following Hamas' incessant rocket fire throughout the humanitarian window, which was agreed upon for the welfare of the civilian population in Gaza, the IDF will now resume its aerial, naval and ground activity in the Gaza Strip," the IDF said.
A poll of 504 Hebrew-speaking Israelis suggests that an overwhelming majority want the offensive to continue, the Jerusalem Post reported Sunday.
It said the poll, sponsored by a political strategist who used to work with Netanyahu, indicated 86.5% responded that Israel shouldn't accept a cease-fire.
Official: Palestinian leaders headed to talks
Palestinian Authority President Mahmoud Abbas is forming a delegation that would head to the Egyptian capital, Cairo, to engage in more diplomacy in the Mideast conflict, said Mohammad Shtayyeh, a senior adviser to Abbas.
Abbas is waiting for a final answer on the initiative from Hamas leaders, who were meeting in Qatar, he said.
Israel has a "hidden agenda," Shtayyeh said.
"Israel wanted to keep Gaza separate from the rest of the Palestinian territory," he said.
He said Israeli troops occupy 50% of Gaza and should withdraw. Not doing so endangers chances for a cease-fire, he said.
Kerry continues diplomatic efforts
U.S. Secretary of State John Kerry continued to make calls Sunday on how to reach a cease-fire, an administration official told CNN.
Kerry returned to the United States early Sunday morning from Paris after his diplomatic efforts aimed at getting a seven-day cease-fire failed.
Washington now believes the best option is to get a cease-fire one day at a time and to get broader talks going in Cairo, with Egypt playing the role of mediator.
"You have a way now to staunch the bleeding," said another U.S. official.
U.N. Secretary-General Ban Ki-moon called Sunday for Israel and Hamas to halt hostilities for an additional 24 hours.
After Sunday's Angelus prayer in Vatican City, Pope Francis was emotional while calling for the end to hostilities in Gaza, Iraq and Ukraine.
"Above all, I think of the children, those who have been denied hope of a decent life, of a future: dead children, wounded children, maimed children, orphaned children, children who have remnants of war as toys, children who don't know how to smile," he said. "Please stop. I ask you with all my heart, it's time to stop. Please stop."
U.N.: 'Please don't go back on the streets'
It is now up to Israel to decide whether it wants to accept the "humanitarian pause," U.N. envoy Robert Serry, who is working around the clock for a pause in violence, said before Netanyahu's comments.
"I'm extremely concerned after both rocket fire and of course also Israeli operations are continuing. And I appeal on both sides to now show utmost restraint for this humanitarian pause to become effective, I hope as soon as possible," Serry said. "This will allow civilians to resume their daily lives, both in Israel and in Gaza."
Serry appealed to both sides "not to miss maybe this last opportunity for calm." He urged Gazans to stay home until there's a durable cessation in violence.
Serry, a Dutch diplomat, is the U.N. special coordinator for the Middle East peace process.
"Please don't go back on the streets or to your houses now until there is an effective cease-fire and an announcement to that effect is made," he said on CNN. "It is still very dangerous for people in Gaza."
The passions of the conflict, meanwhile, echoed across the world.
About 40 protesters were arrested in Paris on Saturday when a banned pro-Palestinian demonstration turned violent, authorities said.
Police brandished shields as they faced off with protesters in a cloud of tear gas at Place de la Republique, a busy pedestrian square in central Paris. Angry protesters hurled glass shards and rocks at police, set a small fire and smashed a bus shelter.
Israel launched its ground incursion in Gaza 10 days ago with the stated aim of taking out the threat posed by tunnels, which run under the border and have been used by militants to carry out attacks on Israeli soil. An Israel military spokesman said it costs about $1 million to build each tunnel.
IDF spokesman Lerner said the Israeli military had exposed 31 tunnels. Israeli forces on Sunday detonated explosives in two tunnels and caused extensive damage to one.
The IDF said Saturday that many Gaza residents were returning to previously evacuated areas despite repeated warnings, placing themselves at risk. It said operations against the tunnel threat continued and defensive positions were being maintained.
Palestinians found more than 100 bodies in areas that have been too dangerous to enter in recent days because of Israeli bombardment, Dr. Ashraf al-Qedra from the Gaza Ministry of Health told CNN.
At least 1,032 Palestinians have been killed and 6,233 wounded since the Israeli operation against Hamas in Gaza started on July 8, al-Qedra said.
Earlier, he had given a higher number, but because bodies are not always intact, the number was reduced after some limbs and other parts were matched with bodies previously accounted for in the total.
The U.N. says 73% of those killed were civilians and 200 were children.
The Israeli operation started with airstrikes, and a ground incursion in Gaza followed on July 17.
Israel blamed Hamas for civilian casualties resulting from Israeli strikes, saying militants have embedded themselves among the civilian population.
"The IDF targets terrorist centers, but if residents are inadvertently hit, it is Hamas which is responsible given that it has -- again -- violated the humanitarian truce that Israel acceded to," Netanyahu's media adviser said.
The IDF said Sunday that the one soldier killed overnight brought to 43 the number of Israeli troops killed in the Gaza operation. Two Israeli civilians have been killed.
CNN's Karl Penhaul reported from Gaza; Steve Almasy and Joe Sterling reported and wrote from Atlanta. CNN's Erin McPike, Chelsea J. Carter, Jethro Mullen, Ray Sanchez, Yousuf Basil, Salma Abdelaziz, Ben Wedeman, Elise Labott, Richard Roth, Ian Lee, Tal Heinrich, Tim Lister, Anna-Maja Rappard, Amir Tal and Samira Said contributed to this report.