(CNN) -- Kofi Annan, the United Nations-Arab League envoy to Syria, told the U.N. Security Council that he was "gravely concerned at the course of events" after Syrian President Bashar al-Assad failed to withdraw troops from cities and towns by Tuesday's self-imposed deadline.
In a letter, Annan said the Syrian government should have used the days ahead of the deadline to send a "powerful political signal of peace."
"In the last five days it has become clear that such a signal has yet to be issued," Annan said. "The cessation of violence in all its forms is a first, but essential, step. It must not be delayed by new conditions. Violence must stop now."
Annan wrote the letter as Syrian troops pounded cities across the nation, opposition activists said. Annan said he was not giving up on the peace plan he brokered, but the fresh violence as the deadline came and went blighted hopes for success.
At least 101 people were killed Tuesday; most of them in the besieged city of Homs, the opposition Local Coordination Committees of Syria said. Tuesday's toll continued a bloody surge in recent days, as the deadline for the troop withdrawal and Thursday's deadline for a cease-fire approached.
The Syrian government said it had taken steps to stand down, but Annan said the military had "conducted rolling military operations in population centers, characterized by troop movements into towns supported by artillery fire."
"While some troops and heavy weapons have been withdrawn from some localities, this appears to be often limited to a repositioning of heavy weapons that keeps cities within firing range," Annan said. "Furthermore, several new localities also appear to have been subject to military operations, including the use of heavy weapons on population centers."
Earlier, Annan visited refugee camps in Turkey, an experience he called "heart-wrenching." More than 24,000 Syrians have crossed the border.
At the Yayladagi camp, resident Mohamed El Abd said refugees frustrated with the peace process hurled insults at Annan, yelling "Liar! Liar!"
U.S. Sens. John McCain and Joe Lieberman also visited Syrian refugees Tuesday and expressed concern for a failing peace process. The two lawmakers, wearing scarves made from the Syrian rebel flag, strolled through the grounds of a former cigarette factory on the Turkish border with Syria, picking up children and shaking hands.
During the tour, the refugees welcomed the lawmakers and chanted "Arm the Free Syrian Army!"
"The only way to reverse this situation is to help the Syrian opposition change the military balance of power on the ground," McCain said. "This means delivering all the nonlethal assistance that has been pledged. But it means doing much more.
"It means helping them establish safe havens to better protect themselves," he said, "and it means regional and international military efforts to defend these safe havens. Will the world continue to stand by while Assad kills thousands and thousands of people?"
Annan refused to say diplomacy was dead, and again urged government and opposition forces to lay down their arms by Thursday, the deadline for a cease-fire agreed upon in the plan.
"If you want to take it off the table, what would you replace it with?" he asked. "Let me once again appeal to the Syrian government, to all the parties, to cease violence according to the plan," Annan said. "And I stress there should be no preconditions."
But both sides spoke of preconditions Tuesday.
Al-Assad's regime had agreed to the deadline but demanded that its opponents, whom it calls armed terrorists, put down their weapons. Damascus also wanted a promise from foreign governments not to fund opposition groups.
Russian Foreign Minister Sergey Lavrov said the Syrian government could have been more active in implementing the peace plan, but he urged heightened international pressure on opposition groups.
"We cannot overlook the fact that Kofi Annan's proposals have not been agreed to by some of the opposition groups, including the well-known Syrian National Council," Lavrov said.
French Foreign Minister Alain Juppe said Annan's letter "confirms the point to which Damascus is flouting its commitments and is adopting a confrontational stance toward its people and the international community."
Annan's description of events was clear, he continued. "Not only has the use of heavy weapons not ended, not only have releases of political prisoners been minimal compared with the dimensions of the crackdown, not only is Damascus now attacking its neighbors, but what was presented as a withdrawal is in fact only a thinly disguised redeployment."
In unusually blunt language, he took aim at the Syrian president. "Bashar al-Assad has lied to Kofi Annan, who enjoys the international community's full support. I regret it all the more in that the opposition was prepared to do its part, provided that Damascus respected its prior commitments."
Sheikh Anas Airout, a senior member of the Syrian National Council, said Annan did not get an agreement from the opposition body but that the Free Syrian Army, a main armed opposition group composed primarily of defected soldiers, committed to abide by the cease-fire as long as the regime stopped its attacks.
Activists in three separate provinces said the Free Syrian Army had held to a 48-hour cease-fire starting Tuesday morning to allow al-Assad the opportunity to withdraw his soldiers.
But with the regime and rebel fighters both refusing to back down until the other side does, prospects faded for an end to the fighting. And the bloodshed continued unabated in cities like Homs, Idlib and suburban Aleppo and Damascus.
Syrian activists had angry words for Annan, saying the peace process was just giving al-Assad more time to kill.
Shells rained down on the western city of Homs, activists said, Rockets destroyed a civilian hospital in the suburbs, and tanks rolled down streets.
"What Annan said is funny and sad at the same time. While rockets and shells are raining down on us, he is talking about talk," activist Abu el Fadaa said. "If these are the requirements of negotiations, we don't want negotiation after every negotiation."
And in the northern province of Aleppo, al-Assad's forces shelled the town of Maarei from its outskirts, the opposition Syrian Observatory for Human Rights said.
"So far, we have not documented until now any withdrawal or movements by the Syrian security forces on the ground, despite the calm that is seen in most parts (of the country)," the group said.
Susan Rice, the U.S. ambassador to the United Nations, said the Security Council members were "unified in their grave concern that this deadline has passed and the violence has not only continued but, over the last 10 days, has intensified."
Early last week, the Syrian government agreed to Tuesday's deadline to withdraw troops. Annan's six-point peace plan also calls for a halt in violence by both sides and a Syrian-led political process to end the crisis.
CNN cannot independently verify reports of violence and deaths as the government has severely restricted access by international media.
The United Nations estimates that the violence in Syria has killed at least 9,000 people. The LCC puts the toll at more than 11,000.
The Syrian regime has consistently blamed the violence in the country on "armed terrorist groups," but many world leaders have said the government is lethally cracking down on dissidents seeking democracy and an ouster of al-Assad, whose family has ruled Syria for 42 years.
Rebels, including defectors from al-Assad's forces, have taken up arms, but they are outnumbered by better equipped government forces.
On Monday, the crisis spilled over into Turkey, resulting in casualties at a refugee camp.
Turkish Prime Minister Recep Tayyip Erdogan said Tuesday that Syria committed a border violation and that officials were "conducting diplomacy with regional countries and will soon take the necessary steps," Turkey's semi-official Anadolu news agency reported.
But Erdogan said Turkey will not turn its back on Syrian refugees.
"We cannot shut down our doors to these people," Erdogan said, according to Anadolu. "If the doors are shut down, they will be toasted."
CNN's Amir Ahmed, Ivan Watson, Yousuf Basil, Holly Yan, Salma Abdelaziz, Moni Basu, John King and Joe Vaccarello contributed to this report.