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.