Arab leaders are gathering to discuss a UN-backed peace plan for Syria at the first major international summit to be hosted by Baghdad in decades.
The UN-Arab League plan would see a UN-monitored end to fighting, troops pulled out of opposition areas and access for humanitarian services.
Syria agreed to the initiative on Tuesday but violence has continued.
Washington has urged countries to maintain pressure on the government of President Bashar al-Assad.
An all-day "curfew" is in place for the summit in Baghdad, the BBC's Wyre Davies reports from the city.
Only about seven of the Arab League's 22 leaders have turned up for the summit, which is being held in such tight security that the venue was not disclosed to journalists.
While expectations are not high for the talks, the fact that they are being held in the Iraqi capital at all can be seen as a sign of progress for Iraq, our correspondent adds.
Earlier, the head of the UN, Ban Ki-moon, urged Syria "to put commitments into immediate effect". "There is no time to waste," he said.
Mr Ban is due to meet key leaders at the summit to discuss how the UN can work with the Arab League to put the plan, brokered by UN envoy Kofi Annan, into action.
The US state department said it 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," a spokesperson said.
Syria has said it will not address any initiative from the Arab League, from which it was suspended last year.
The opposition in Syria is sceptical about the terms of Mr Annan's plan, with some saying Mr Assad is merely stalling 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."
The UN says more than 9,000 people have been killed during the year-long Syrian revolt.
Opposition activists say at least 40 people have been killed since government troops overran the opposition-held town of Saraqeb in the north-west at the weekend.
Corpses littered the streets as homes were burned to the ground and shops pillaged and looted, they said in reports which could not be verified independently.
Security is extremely tight for the Baghdad summit and much of the city has been brought to a standstill for the summit, which is costing an estimated $500m (£314m) to stage.
Many Arab leaders have chosen not to attend. This will disappoint the Iraqi government which is hoping to re-establish itself in the Arab fold after years of violence and sectarian conflict, our correspondent says.
Little progress is expected either on the Syrian front or on wider tensions between Shia and Sunni factions in the region, he says.
But if the summit, which is expected to last for barely a few hours, passes off without incident and if there are no insurgent attacks elsewhere in the country, it will be seen as a resounding success, our correspondent adds.