The United States and Russia have hailed a breakthrough deal to put Syria's peace process back on track, but some rebel factions say they doubt it will hold as clashes continue in Aleppo.
The agreement reached by the powers backing opposing sides in the conflict promised a Syria-wide truce effective from sundown on Monday, improved humanitarian aid access and joint military targeting of banned militant groups.
But only hours after it was announced, Syria's army attacked rebel-held areas in the northern city of Aleppo, both sides said, as the military pushed to maximise gains before the ceasefire deadline.
Insurgents said they were planning a counter-offensive.
"The fighting is flaring on all the fronts of southern Aleppo but the clashes in Amiryah are the heaviest," said Abdul Salam Abdul Razak, spokesman of the rebel Nour al-Din al Zinki Brigades.
Syria's five-year conflict has killed thousands in Aleppo, the main focus of clashes between insurgents, including foreign-backed rebels, and pro-government forces backed by Russia and Iran.
US Secretary of State John Kerry called on all sides to respect the deal, which was reached after marathon talks in Switzerland and several failed attempts to hammer out the details in recent weeks.
"This requires halting all attacks, including aerial bombardments, and any attempts to gain additional territory at the expense of the parties to the cessation. It requires unimpeded and sustained humanitarian access to all of the besieged and hard-to-reach areas including Aleppo," he said.
Russian Foreign Minister Sergei Lavrov said that despite continuing mistrust, the two sides had developed five documents that would enable co-ordination of the fight against terrorism and a revival of Syria's failed truce in an enhanced form.
Both sides agreed not to release the documents publicly.
"This all creates the necessary conditions for resumption of the political process, which has been stalling for a long time," Lavrov told a news conference.
Kerry said the "bedrock" of the new deal was an agreement that the Syrian government would not fly combat missions in an agreed area on the pretext of hunting fighters from the al-Nusra Front.
Under the agreement, Russian-backed government forces and opposition groups, supported by the United States and Gulf countries, would halt fighting for a while as a confidence-building measure.
If the truce holds from Monday, Russia and the United States will begin seven days of preparatory work to set up a "joint implementation centre", where they will share information to delineate territory controlled by the al-Nusra Front and opposition groups.
Both warring sides would pull back from the strategic Castello Road in Aleppo to create a demilitarised zone, while opposition rebels and government groups would both have to provide safe and unhindered access via Ramouseh in the south of the city.
The opposition Free Syrian Army rebels said on Saturday they were sceptical the deal would be enforced as Damascus and Moscow had continued bombing their areas under earlier truces.