Kofi Annan, the UN-Arab League envoy to Syria, has said it is not too late to implement a UN-brokered peace plan, despite reports of continued violence across the embattled country.
Activists on Tuesday reported heavy shelling by government troops in the city of Homs and the northern village of Marea in Aleppo.
The Syrian Revolution General Commission, an activist network, said that at least 97 people were killed in violence across the country.
In the northern Idlib and central Hama provinces, troops backed by helicopters were firing heavy machine guns in an attempt to flush out opposition fighters, activists said.
The six-point peace plan, brokered by Annan, requires Syrian forces to pull back from towns and villages on Tuesday and both sides to cease all hostilities on Thursday morning.
The former UN chief said he believed it was too early to say that the plan has failed.
"The plan is still on the table and is a plan we are all fighting to implement," Annan said.
“It is a plan the [UN Security] Council has endorsed. It's a plan the Syrians have endorsed and from the comments made by the opposition, they are also prepared to go along with it if the government meets its commitments to pull the troops out. So I think the plan is very much alive."
His comments came prior to his arrival in the Iranian capital for talks on Syria.
Earlier on Tuesday, Walid Muallem, the Syrian foreign minister, said in Moscow that Damascus had started pulling some troops out of different provinces.
"I told my Russian colleague of the steps Syria is taking to show its goodwill for the implementation of the Annan plan," Muallem said after talks with Sergei Lavrov, the Russian foreign minister.
But Annan said he had information that while the Syrian military was withdrawing from some areas, it was also moving to others not previously targeted.
Bernard Valero, French foreign ministry spokesman, dismissed Syria's claims of a withdrawal as "a new expression of this flagrant and unacceptable lie".
William Hague, British foreign secretary, accused Damascus of using the cease-fire deadline "as a cover for intensified military efforts to crush Syria's opposition".
The White House also said that Syria is not abiding by the plan, adding that it would work with international partners on "next steps" against Damascus if it failed to meet its commitments.
"We have seen much evidence of further brutality and oppression against innocent civilians," White House spokesman Jay Carney told reporters aboard Air Force One headed for the US state of Florida.
For their part, Russia and China, who have twice shielded Syria from UN sanctions, called on the government of Bashar al-Assad as well as the opposition to work towards Annan’s proposed truce.
Lavrov said Syria's government "could have been more active and decisive'' in implementing the peace plan.
Separately, Liu Weimin, the Chinese foreign ministry spokesman, said his country hopes the Syrian government and opposition “stay with their commitment to the ceasefire and withdrawal, and create favourable conditions for easing the tension in Syria and pushing for a political resolution for the Syria issue".
The main Syrian opposition group, the Syrian National Council, estimated that some 1,000 people have been killed in regime attacks in the week leading up to Tuesday's withdrawal deadline.
Assad's crackdown on the uprising, that has brought clashes with an increasingly armed opposition, has killed more than 9,000.