The US has increased pressure on Syria to implement a UN-Arab League peace plan, as Arab leaders gather for talks in Baghdad.
The plan, brokered by Kofi Annan, calls for a UN-monitored end to the fighting, the pull-out of troops from opposition cities and humanitarian access.
Syria agreed to the initiative on Tuesday, but violence has continued.
The issue is set to top the agenda in Baghdad, which is hosting its first major summit in more than 20 years.
The BBC's Wyre Davies says expectations are not high, but the mere fact the Arab League is meeting in the Iraqi capital at all can be seen as a sign of progress for the country.
Earlier, the head of the UN, Ban Ki-moon, urged Syrian President Bashar al-Assad "to put commitments into immediate effect".
"There is no time to waste," he said, describing the plan as an "initial step forward".
Mr Ban is due to meet key leaders at the summit to discuss how the UN can work with the Arab League to put Mr Annan's proposal into action.
Speaking on Wednesday, US State Department spokeswoman Victoria Nuland said they had "not seen the promises that Assad made implemented".
"It's incumbent on all of us to keep the pressure on Assad to meet the commitment that he's made, and that's our intention over the next few days."
On Tuesday, Mr Annan announced that Syria had accepted his six-point peace deal.
But Syria has also said it will not address any initiative from the Arab League, from which it was suspended last year.
On Wednesday, a Syrian government spokesman told the BBC: "Since our membership was suspended, we deal with other Arab countries on a bilateral level only.
"Therefore, we will not be dealing with or addressing any initiative that comes out of the Arab League at any level," Jihad Makdessi said.
Syria's opposition is sceptical about the terms of Mr Annan's plan, with some saying Mr Assad's acceptance of it was merely a stall for time, in order to continue his crackdown.
"We are not sure if it's political manoeuvring or a sincere act," said Louay Safi, a member of the opposition Syrian National Council. "We have no trust in the current regime. ... We have to see that they have stopped killing civilians."
Security is extremely tight for the Baghdad summit and the city has been brought to a standstill ahead of the summit, which is costing an estimated $500m (£314m) to stage.
But many Arab leaders have chosen not to attend, a move that will disappoint the Iraqi government which is hoping to re-establish itself into the Arab fold after years of violence, says our correspondent.
Meanwhile, violence has continued in Syria, with the army bombarding a town in the province of Hama for the 17th straight day, activists said.
Government troops stormed the rebel bastion of Qalaat al-Madiq, said the UK-based Syrian Observatory for Human Rights.
Shelling also continued around Homs, the scene of a major offensive by government forces last month, activists said.