International envoy Kofi Annan said that the violence in Syria must end "with no preconditions" after President Assad's regime continued shelling residential neighbourhoods despite a deadline to withdraw.
Speaking in Turkey, Annan claimed that his peace plan, endorsed by the United Nations security council, had to be given more time to work.
"We still have time between now and 12 April to stop the violence," he said. "If you want to take [the peace plan] off the table, what would you replace it with?"
Even as Syrian foreign minister Walid al-Moualem claimed the regime had begun withdrawing troops from Syrian cities, violence was reported in several locations.
At least 11 people have been killed in various locations on Tuesday, activists said.
The Syrian National Council, based in Turkey, said that up to 1,000 people had been killed in the past 8 days, most of them civilians.
Activists reported shelling in Homs on Tuesday, as well as the presence of tanks in Hama.
One Homs resident told Reuters news agency that the shelling began at 8.30am local time, and that "I can now hear one shell every 10 minutes or so".
The White House said it had seen no credible evidence of the Syrian security forces withdrawing.
"We have seen much evidence of further brutality and oppression against innocent civilians," White House spokesman Jay Carney told reporters on Air Force One.
Earlier, British foreign secretary William Hague said: "There is no evidence so far that the Assad regime has any intention of adhering to any agreement it makes.
"We will be ready to intensify our support for the Syrian Opposition, and to support others seeking to do the same. And we will begin the process of seeking the referral by the Security Council of the situation in Syria to the International Criminal Court."
The government had agreed to withdraw its forces from civilian areas by Tuesday under the plan drawn up by UN and Arab League special envoy Kofi Annan.
It was hoped that a full ceasefire would be in place within 48 hours.
But fierce fighting on Monday, which saw 150 people reported killed and shots fired across Syria's borders with Lebanon and Turkey, continued through Tuesday, including in areas previously untouched by the crackdown.
The Syrian foreign minister Walid Muallem was in Russia on Tuesday for talks as its ally was said to be growing frustrated with the lack of progress.
At a meeting in Moscow with Russian foreign minister Sergei Lavrov, Muallem said "despite all [our] positive measures we noticed on a daily basis the escalation of opposition by the armed terrorist gangs".
Lavrov said Syria could have been more "resolved" in its peace efforts, but said the opposition also needed to do more.
China also reiterated its calls for both sides of the conflict to agree to and implement a full ceasefire.
According to the UN more than 9,000 people have been killed in Syria since the uprising against Assad's government began in March 2011.