Jerusalem (CNN) -- Israeli forces struck targets in Rafah and Khan Yunis in Gaza on Friday, killing 12 people and injuring at least seven, Palestinian sources said.
Two Palestinian civilians were killed in an artillery strike east of Gaza City. A Hamas militant, wounded in an aerial strike earlier Friday, also died of his wounds, they said.
An airstrike in Rafah south of Gaza killed three people, including two top commanders of the military wing of Hamas, the group said. The third victim was one of the officials' bodyguards. Two other people were injured in the attack.
More airstrikes were reported in the Gaza neighborhoods of Zeitoun, but no injuries were reported.
With the latest strikes, at least 17 Palestinians have been killed since a Hamas attack on an Israeli bus critically injured a teenage boy Thursday afternoon.
On Friday, Hamas Interior Ministry spokesman Ihab al-Ghusseini accused Israel of dropping three bombs in central Gaza on Thursday that contained white phosphorous -- an incendiary weapon.
Israel says the claim is unfounded.
Israel's military said the attacks targeted Hamas militants but said one of the strikes appeared to have also injured "uninvolved civilians."
Two Hamas militants were killed and one injured in an Israeli airstrike east of Khan Yunis, according to security and medical sources.
In another strike, a 50-year-old woman, her daughter, 18, and a 60-year-old man died when a tank shelled a home east of Khan Yunis. Four people were injured.
Israeli warplanes also attacked a tunnel in Rafah. No deaths were reported, but black smoke billowed from the site. Tanks continued shelling targets in Gaza as helicopter gunships and drones flew overhead.
Hamas security sources say that an airstrike north of Gaza killed a 27-year-old man named Raed Shehade, who headed a rocket unit in a beach camp area run by the Izzedine al Qassam brigades, the military wing of Hamas.
Confirming the strikes on Friday morning, the Israel Defense Forces said it "will not allow any attempt to harm Israeli civilians" and "holds the Hamas terrorist organization solely responsible for any terrorist activity emanating from the Gaza Strip."
"The IDF regrets that the Hamas terrorist organization chooses to operate from within its civilian population, using it as a 'human shield,'" the military said in a statement.
Fifteen mortars and rockets were launched from Gaza into Israel on Friday morning, according to Israeli police. One landed inside a kibbutz, causing some damage, police said.
One of the rockets was intercepted by Israeli missile defense systems, while four others landed in the city of Ashkelon in southern Israel.
No injuries were reported.
A spokesman for the Izzedine al Qassam brigades in Gaza later took credit for the attack.
U.N. Secretary-General Ban Ki-moon urged Hamas to stop firing rockets immediately and called for Israel to exercise "maximum restraint."
"He urges respect for international humanitarian law and calls for de-escalation and calm to prevent any further bloodshed," the United Nations said in a statement Friday.
After a meeting with Israeli President Shimon Peres, the office of the secretary-general issued a statement saying the two had "discussed the worrying impasse in the Middle East peace process and the urgent need for a way forward."
Hamas controls the Palestinian government in Gaza. Listed as a terrorist organization by the United States, Hamas has carried out dozens of terrorist attacks, killing Israelis and others.
A Hamas spokesman in Gaza, Fauzi Barhum, said, "The resistance movement's response to the enemy's massacre comes as self-defense, and to protect the citizens. It aims to pressure the occupier to stop committing crimes."
Telal abu Rahme in Gaza contributed to this report