The Syrian government and the main opposition umbrella group say they accept the terms of a deal to cease hostilities from Saturday.
The government said it would halt “combat operations” in line with the plan announced by the USA and Russia.
But the opposition said its acceptance depended on government forces ending sieges and air strikes of civilians.
The deal will not apply to the two main jihadist groups in Syria, Islamic State (IS) and the rival al-Nusra Front.
Al-Nusra is an affiliate of al-Qaeda and forms part of a major rebel alliance that controls large parts of the country’s north-west.
More than 250,000 people have died in almost five years of war in Syria.
Eleven million others have fled their homes as forces loyal to President Bashar al-Assad and those opposed to his rule battle each other, as well as IS militants.
“Right to respond”
Under the terms of the agreement announced by the USA and Russia on Monday, the Syrian government and opposition were required to indicate by noon on Friday (10:00 GMT) whether they would comply with the cessation of hostilities.
The High Negotiation Committee (HNC) issued a statement after a meeting in Saudi Arabia saying it was “committed to the success of the international efforts dedicated to ending Syrian bloodshed”.
But it warned that “acceptance of the truce is conditional” on the implementation of UN Security Council resolution 2254, which calls on all parties to lift sieges, allow aid deliveries, halt aerial and artillery attacks on civilians, and release detainees.
The Syrian government later declared “its acceptance of a halt to combat operations on the basis of continuing military efforts to combat terrorism”.
It added that it would work with Russia, which has conducted air strikes against Mr Assad’s opponents since September, to identify areas and armed groups that were covered and reserved the right to “respond to any breach by these groups “
The Syrian government also stressed the importance of sealing Syria’s borders, halting foreign support for armed groups, and “preventing these organisations from strengthening their capabilities or changing their positions”.
The Kurdish-dominated Syrian Democratic Forces (SDF) alliance told the Associated Press that it would not abide by the deal because it was fighting only IS in northern Syria.
The UN special envoy to Syria, Staffan de Mistura, has described the plan for the cessation of hostilities as encouraging, but acknowledged that enforcing it on the ground will be challenging.
Mr de Mistura will convene a taskforce to monitor the deal as soon as it takes effect, due at midnight on Saturday (22:00 GMT on Friday).
The USA, which supports the opposition to Mr Assad, will also share information with Russia, including data that delineates territory where armed groups are active.
Rebel commanders expressed doubts about the accord, saying it would provide cover for government forces and Russian aircraft to continue their attacks.
“Russia and the regime will target the areas of the revolutionaries on the pretext of al-Nusra Front’s presence,” Bashar al-Zoubi of the Yarmouk Army told the Reuters news agency. “If this happens, the truce will collapse.”
US Secretary of State John Kerry told a Senate committee that it would be clear very soon whether Russia was serious about ending the conflict.
“The proof will be in the actions that come in the next days,” he said.
He added that Mr Assad was going to “going to have to make some real decisions” about the formation of a transitional administration, as part of a UN Security Council-endorsed peace process, or the USA would consider “Plan B options”.
Fighting continued on Tuesday despite the plan for a cessation of hostilities.
The Syrian Observatory for Human Rights, a UK-based monitoring group, reported that air strikes, believed to have been carried out by Russian jets, had targeted one of the last roads into opposition-held eastern areas of the city of Aleppo.
South of Aleppo, IS militants captured the town of Khanaser, cutting a road used to supply government-controlled areas of the city, the Syrian Observatory said.
Meanwhile, state TV reported that an aid convoy had entered the eastern Damascus suburb of Kafr Batna, which is under siege by government forces.