The US and Russia have announced that a planned cessation of hostilities in Syria will come into effect at midnight on 27 February.
Their statement said the truce did not include so-called Islamic State (IS) and the al-Qaeda-linked Nusra Front.
World powers agreed on 12 February that a truce would come into effect within a week, but that deadline passed and scepticism remains over the new plan.
On Sunday 140 died in bombings in Homs and Damascus as the violence continued.
More than 250,000 Syrians have been killed in the conflict which began in March 2011.
Some 11 million others have been forced from their homes, of whom four million have fled abroad – including growing numbers who are making the dangerous journey to Europe.
History of the conflict – how the civil war has spread
Mapping the war – the shifting territorial gains
Separately, Syria’s government has called a parliamentary election for 13 April. The last was in 2012 and they are held every four years.
The White House said that President Barack Obama had phoned his Russian counterpart Vladimir Putin at Mr Putin’s request to discuss the efforts to establish the cessation of hostilities.
After their phone call, the joint Russian-US statement was released.
The truce applied to “those parties to the Syrian conflict that have indicated their commitment to and acceptance of its terms”, the statement said.
This excluded IS, Nusra and “other terrorist organisations designated by the UN”.
Air strikes by Syria, Russia and the US-led coalition against these groups would continue, the statement read.
It said that armed opposition groups taking part would have to confirm their participation by midday on 26 February.
Russian and Syrian planes would halt any attacks on the armed opposition groups.
Russia and the US will work together to “delineate territory where groups that have indicated their commitment to and acceptance of the cessation of hostilities are active”.
The deal also sets up a communications hotline and calls for a working group to monitor ceasefire violations.
Syria’s main opposition grouping, the High Negotiations Committee, said it would accept the truce, but its commitment was conditional on the lifting of sieges, an end to attacks on civilians, the freeing of prisoners and the delivery of aid.
US Secretary of State John Kerry welcomed the deal, saying: “If implemented and adhered to, this cessation will not only lead to a decline in violence, but also continue to expand the delivery of urgently needed humanitarian supplies to besieged areas.”
Russia and the US back opposing sides in the war; Moscow is President Bashar al-Assad’s strongest ally.
On Saturday, President Assad had said he would be ready for a ceasefire, if what he termed “terrorists” did not take advantage of the lull in the fighting. But he had previously cast doubt on the success of a truce.