The US and Russia have announced plans for a "cessation of hostilities" in Syria that would take effect on Saturday (local time) but exclude groups such as Islamic State and al-Qaeda's Nusra Front, a loophole Syrian rebels immediately highlighted as a problem.
Monday's agreement, described by a UN spokesman as "a first step towards a more durable ceasefire", is the fruit of intensive diplomacy between Washington and Moscow, which back opposing sides in the five-year-old civil war that has killed more than a quarter of a million people.
Presidents Barack Obama and Vladimir Putin discussed the accord by phone, and the Kremlin leader said it could "radically transform the crisis situation in Syria". The White House said it could help advance talks on bringing about political change in Syria.
To succeed, the deal will require both countries to persuade their allies on the ground to comply.
It allows the Syrian army and allied forces, as well as Syrian opposition fighters, to respond with "proportionate use of force" in self-defence. And it leaves a significant loophole by allowing continued attacks, including air strikes, against Islamic State, Nusra and other militant groups.
Bashar al-Zoubi, head of the political office of the Yarmouk Army, part of the rebel Free Syrian Army, said this would provide cover for Syrian President Bashar al-Assad and his Russian allies to continue to attack opposition-held territory where rebel and militant factions are tightly packed.
"Russia and the regime will target the areas of the revolutionaries on the pretext of the Nusra Front's presence, and you know how mixed those areas are, and if this happens, the truce will collapse," he said.
Since intervening with air strikes in support of Assad in September, Russia has helped pave the way for significant advances by government forces in a conflict that has sucked in a host of world and regional powers.
The Syrian army is backed by Moscow, Iran and fighters from Lebanon's Hezbollah; ranged against them are rebels supported by the US, Turkey and Saudi Arabia.
A joint US-Russian statement said the two countries and others would work together to delineate the territory held by Islamic State, Nusra Front and the other militant groups excluded from the truce.
But rebel officials said it was impossible to pinpoint positions held by Nusra.
"For us, al-Nusra is a problematic point, because al-Nusra is not only present in Idlib, but also in Aleppo, in Damascus and in the south. The critical issue here is that civilians or the Free Syrian Army could be targeted under the pretext of targeting al-Nusra," said a senior opposition figure, Khaled Khoja.
The main opposition body, the High Negotiations Committee, was convening a meeting in the Saudi capital Riyadh on Monday to discuss the truce.
In a sign of confidence, reflecting his growing momentum on the battlefield, Assad on Monday called a parliamentary election for April 13. The timing was not a surprise as elections are held every four years and the last one was in 2012.
UN Syria envoy Staffan de Mistura told Reuters the cessation accord could allow a resumption of negotiations. "We can now relaunch very soon the political process which is needed to end this conflict," he said.
Under the terms of the cessation, parties would indicate their agreement to the US and Russia by noon on Friday (Damascus time), and the truce would go into effect at midnight, the two countries said.
Syrian government and allied forces will cease attacks against armed opposition forces, and vice versa, with any weapons including rockets, mortars, anti-tank guided missiles.