Updated 4:14 PM ET
(CBS/AP)BEIRUT - International envoy Kofi Annan implored Syria on Tuesday to halt all violence within 48 hours, after Syrian troops shelled and raided opposition strongholds across the country in violation of his peace plan and put a desperately needed truce at risk.
The Syrian opposition as well as the U.S. and its allies have been deeply skeptical that the regime would comply with the cease-fire because it has violated previous agreements and stepped up attacks in recent weeks.
The main Syrian opposition group estimated that some 1,000 people have been killed in regime attacks in the week leading up to Tuesday's withdrawal deadline. Tuesday's fighting claimed the lives of at least 38 civilians and 19 regime soldiers, activists said.
Syria's Foreign Minister Walid Moallem claimed the regime was complying with the truce deal, saying Tuesday that "we have already withdrawn forces and army units from several Syrian provinces."
But Annan told the U.N. Security Council in a letter that the regime had not pulled troops and heavy military equipment out of cities and towns by Tuesday as promised. He said there's still time to change course and halt all violence by 6 a.m. Thursday, adding that opposition fighters indicated they would observe a cease-fire "provided Syrian forces withdraw from cities," according to a copy of the letter obtained by The Associated Press.
The council strongly backed Annan, with all 15 members -- including Syrian allies China and Russia -- approving a press statement expressing "deep concern" at the failure by Damascus to withdraw its troops and heavy equipment.
"Obviously, members of the council are 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," said Susan Rice, U.S. ambassador to the U.N.
Activists also said Syrian forces carried out new attacks and showed no signs of pulling back.
Residents of Homs reported some of the heaviest shelling in months.
"Hundreds of mortar rounds and shells were falling around all day," resident Tarek Badrakhan told The Associated Press. He said a makeshift hospital housing wounded people and dozens of corpses was destroyed in the shelling.
"It's now on the ground," he said.
Annan, the U.N.-Arab League envoy, expressed disappointment with the lack of compliance but insisted his plan has not failed. "The plan is still on the table and is a plan we are all fighting to implement," he told reporters in Hatay, Turkey, after touring a nearby camp of Syrians who fled to the area.
The envoy said violence must stop without preconditions.
"I had hoped that by now, we would have been much further ahead," he said. He included opposition fighters in his appeal for calm, but directed most of his criticism at the Syrian government, saying that it is "time the military go back to their barracks."
Syrian troops have withdrawn from some areas, but continued to carry out attacks Tuesday, "including the use of heavy weapons on population centers," he wrote in his letter.
Annan also complained that Syria introduced new truce conditions over the weekend, including seeking written assurances that armed groups are prepared to cease all violence, that these groups disarm immediately and that countries in the area not finance or arm rebel groups. Syria had accepted Annan's truce plan last week, and introducing new demands is not acceptable, he said.
Annan said he was assured by Russia that Syria no longer sought written guarantees, but sought truce assurances nonetheless.
From Turkey, Annan flew to Iran in an apparent attempt to get one of Syria's main allies to back his peace plan.
The Syrian opposition has said it does not recognize the Assad regime and will not provide written guarantees.
Options for ending the fighting appear to be dwindling with the international community unwilling to intervene militarily. A collapse of the truce could push Syria even closer to an all-out civil war.
Meanwhile, U.S. Senators John McCain and Joseph Lieberman were also visiting a Syrian refugee camp in Hatay, Turkey. In a joint statement released by McCain's office Tuesday, the two thanked the Turkish people for their generosity towards their Syrian neighbors and added that the international community is failing Syria's citizens.
"Make no mistake: The situation in Syria is an armed conflict. This is a war. Diplomacy with Assad has failed, and it will continue to fail so long as Assad thinks he can defeat the opposition in Syria militarily. And right now, using tanks and artillery and even attack helicopters, Assad has the upper hand on the battlefield."
The statement adds: "The only way to reverse this dynamic is by helping the Syrian opposition to change the military balance of power on the ground. This means delivering all of the non-lethal assistance that has been pledged thus far. But it means doing a lot more. It means a concerted international effort to provide these brave fighters in Syria with the capabilities to defend themselves."
White House press secretary Jay Carney said the U.S. has seen no evidence of a pullback, but "much evidence of further brutality and oppression." French Foreign Ministry spokesman Bernard Valero dismissed Syria's claims of a withdrawal as "a new expression of this flagrant and unacceptable lie." And British Foreign Secretary William Hague accused Damascus of using the cease-fire deadline "as a cover for intensified military efforts to crush Syria's opposition."
Unlike previous peace plans, this one has the backing of Assad allies Russia and China because it did not call for the Syrian leader to step aside ahead of talks on a political transition. Russia and China have shielded Syria from U.N. Security Council condemnation in the past, arguing that only negotiations with the regime offer a way out of the crisis.
But even Russia seemed to be losing patience with Assad's regime, with Russian Foreign Minister Sergey Lavrov complaining that Syria's "efforts to implement the plan could have been more active and resolute."
Lavrov said in a joint news conference with Moallem that he had spoken about it "quite frankly" with Syrian officials. Yet he seemed to place equal blame on the opposition, repeating Russia's call for the West to pressure rebel fighters to halt violence as well.
Lavrov also called for a speedy deployment of international observers -- including Russians -- in the country. Sending U.N. observers currently posted on the Golan Heights, a strategic plateau captured by Israel from Syria in 1967, was being discussed, he said.
Syrian opposition leaders said Tuesday they remain committed to the cease-fire, even though activists in Syria said they've seen no signs of a troop pullback. "Soldiers are not being withdrawn from towns and villages," said Fadi al-Yassin, an activist in the Idlib province close to Turkey. "On the contrary, reinforcements are being sent."
Regime forces have used heavy weapons including anti-aircraft guns against civilians, Bassma Kodmani, a spokeswoman for the largest opposition group, the Syrian National Council, told reporters in Geneva.
Kodmani estimated that some 1,000 people were killed since Annan announced the cease-fire timeline on April 2. "So every day is a very, very painful time that is given to the Assad regime," she said, adding the death toll was based on figures provided by various groups, including the Syrian Arab Human Rights Network.
Burhan Ghalioun, head of the Syrian National Council, called for U.N. protection for civilians in Syria.
Syria restricts the access of foreign journalists, and claims about casualties cannot be verified independently. The U.N. has said previously it believes more than 9,000 people have been killed in the 13-month uprising against Assad.
The Britain-based Syrian Observatory for Human Rights, an activist group, reported shelling attacks and raids in several locations in the north, center and south of the country. It said at least 29 Syrian civilians were killed, along with 11 soldiers shot dead by opposition fighters ambushing checkpoints.
In the northern province of Aleppo, troops fired shells at the town of Mareh, said local activist Mohammed Saeed, adding that earlier in the week several residents of Tal Rifaat, another town in the area, were killed by shelling. A video posted by activist online showed about a dozen charred bodies inside a room said to be in Tal Rifaat.
In northern Idlib and central Hama province, troops backed by helicopters were firing heavy machine guns to try to flush out opposition fighters, said al-Yassin, the activist from Idlib. Regime forces detained residents and set four homes on fire in Idlib's Ariha village and a contingent of 50 army vehicles entered the town of Kfar Zeita in Hama province, he said.
The Observatory said troops also fired shells at two neighborhoods in Homs. Additional raids were reported in two southern villages, the group said. An activist in the Damascus suburb of Douma, said tanks were patrolling the streets, as they have in recent days. Some tanks bore graffiti reading "Assad's shield," he said.
Over the weekend, there were already signs the regime was stalling for time, including a new, last-minute demand for written guarantees that the rebels would lay down their arms.
And it appeared the Assad regime was still seeking to renegotiate terms of the peace plan on the very day they're supposed to be implementing it, CBS News correspondent Clarrisa Ward reports, with Syrian Foreign Minister Moallem raising another new demand - that a cease-fire must start simultaneously with the deployment of an international observer mission. The deployment of observers was one of the terms of Annan's plan.
In another apparent shift, Moallem said Syria wants the truce guarantees to be issued by Annan, not by the opposition fighters.
"We did not ask for guarantees from armed terrorist groups that practice killing, take hostages and destroy infrastructure. We want guarantees from Annan," he said in Moscow.
The uprising against Assad's regime has turned increasingly militarized in response to a brutal regime crackdown. The fighting is also spilling across Syria's borders, raising the risk of a regional conflagration.
Turkish Prime Minister Recep Tayyip Erdogan on Tuesday accused Syria of violating the border when Syrian forces opened fire the day before, killing a TV journalist in Lebanon and two people in a refugee camp in Turkey. He said his country was considering what steps to take in response but did not elaborate on possible measures.
Turkey, which has already given shelter to some 24,000 Syrian refugees, has floated the idea of creating security zones along its border, a step that could drag the Turkish military into the conflict.
Moallem on Tuesday accused Turkey of helping fuel the violence, saying it was hosting gunmen in training camps and allowing them to cross the border and smuggle weapons.
Asked about the possibility of a Turkish buffer zone on the border, he said that "Syria is a sovereign state and has the right to defend its sovereignty against any violation of this sovereignty."