Syrian troops have shelled and raided opposition strongholds nationwide, activists said, prompting an urgent appeal by international envoy Kofi Annan to the Syrian regime to halt violence "without conditions" and give his truce plan a chance.
Syria's foreign minister claimed that regime forces have begun withdrawing from some areas in compliance with Mr Annan's plan, which requires Syrian forces to pull back from towns and villages on Tuesday and both sides to cease all hostilities by 6am on Thursday, local time.
However, activists said Syrian forces carried out new attacks and that there were no signs of a withdrawal.
In a news conference in Hatay, Turkey, Mr Annan insisted that his plan has not failed and that Syria still had time to comply.
"We still have time between now and Thursday 12 to stop violence," he said.
"I appeal to all, the government in the first place (to halt fighting)." He also said that violence must stop without conditions.
Mr Annan said the UN Security Council would take up the issue later today.
The main Syrian opposition group, meanwhile, estimated that some 1,000 people have been killed in escalating regime attacks in the week leading up to the withdrawal deadline, though such figures cannot be verified independently.
France and Britain accused Syria of deception and even Damascus ally Russia seemed critical of Bashar 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".
Syrian opposition leaders said they remain committed to the ceasefire brokered by Mr Annan.
The truce is widely seen as the last chance for diplomacy, and its collapse could push Syria even closer to an all-out civil war.
The opposition as well as the US and its allies have been sceptical that the regime would comply with the ceasefire, because it has violated previous agreements and stepped up attacks in recent weeks. At the same time, options for ending the fighting appear to be dwindling with the international community unwilling to intervene militarily.
British Foreign Secretary William Hague accused Damascus of using the ceasefire deadline "as a cover for intensified military efforts to crush Syria's opposition".
French foreign ministry spokesman Bernard Valero dismissed Syria's claims of a withdrawal as "a new expression of this flagrant and unacceptable lie".
Opposition activists said they have seen no signs of a troop withdrawal. "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."