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 US 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.
Under the terms of the agreement announced by the US 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), which represents most of the main opposition and rebel factions, 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 ".
Moscow's military intervention in Syria has been a game changer.
Not only has it forced the West to sit down at the negotiating table and deal with Russia and its president, but also to recognise Russia as a major world power. And, of course, it has given a huge boost to Syrian President Bashar al-Assad.
The reason Washington no longer demands the Syrian leader's resignation as a pre-condition to peace, is because it knows that is no longer realistic: Russian firepower has made Mr Assad more secure, his armed opponents weaker. Image copyrightAPImage caption Vladimir Putin said the cessation of hostilities could "radically transform" the crisis in Syria
The Russians sense that both America and Europe want a swift end to this conflict, and they know that the Syrian refugee crisis is putting a huge strain on the EU.
So, Russia sees its chance to change the way it is viewed: it wants to be seen by the West not as the problem, but the solution.
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 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 ceasefire taskforce to monitor the deal as soon as it takes effect, due at midnight on Saturday (2200 GMT on Friday).
The US, 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 attacking opposition-held areas.
"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."
Fighting was continuing on Tuesday despite the accord.
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.
Meanwhile, a UN and Syrian Arab Red Crescent aid convoy was expected to deliver desperately needed supplies of food and medicine to the eastern Damascus suburb of Kafr Batna, which is under siege by government forces
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