The United States and Russia reached a deal on a new Syrian ceasefire, which, if it holds, could see the first joint military effort by the two powers against jihadists.
US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov said that the truce, reached late on Friday, would come into force on Monday, the first day of the Muslim holiday of Eid al-Adha.
The two powers back opposite sides of the conflict, with Moscow supporting the regime of President Bashar al-Assad and the US behind a coalition of rebel groups it regards as moderate.
But if Russia is able to pressure Assad to respect the ceasefire for a week, Moscow and Washington will set up a joint coordination unit and begin air strikes against agreed "terrorist" targets.
"We will jointly agree on strikes against terrorists to be carried out by the Russian and American air forces. We have agreed on the zones in which these strikes will be carried out," said Lavrov.
The much anticipated — if tentative — breakthrough came at the end of marathon talks between Lavrov and Kerry in Geneva, as the pair push for an end to the five-year civil war that has killed 290,000 and displaced half the country\’s population.
"Today, the United States and Russia are announcing a plan which we hope will reduce violence, ease suffering and resume movement towards a negotiated peace and a political transition in Syria," Kerry said.
The vexed question of Assad\’s fate remains, with Western powers calling for his removal and Russia backing him.
But both Kerry and Lavrov said the complex plan represents the best available chance to end the fighting between the regime and the mainstream opposition rebels, while still targeting jihadists from former Al-Qaeda affiliate Al-Nusra Front and the Islamic State group.
Key to the deal is the withdrawal of Syrian regime forces around rebel-held Aleppo, allowing desperately needed humanitarian access to besieged communities.
Russia also needs to persuade the Syrian air force to stop strikes on anti-government positions, which have killed large numbers of civilians.
In turn, Washington has to get the opposition groups it backs to separate themselves from the former Al-Nusra, now called Fateh al-Sham Front, which has allied itself with a range of rebels at different points in the fluid conflict.
"The armed opposition in Syria now faces what is perhaps its biggest and most momentous decision since they chose to take up arms against the Assad regime in 2011," said Charles Lister, a senior fellow at the Middle East Institute think-tank.
Mainstream rebels appear reluctant to withdraw from frontlines where Fateh al-Sham Front jihadists are also present because of fears the ceasefire will fail, he said.
"For this reason alone, many opposition figures see the US-Russia talks and whatever comes from them as a conspiracy against their long and hard fought for revolution. It will be hard to change this mindset."
Only if commitments by Moscow and the Assad regime to cease violence for seven days are "fully met" will the US and Russia start cooperating with joint strikes, the Pentagon said in a statement following the announcement.
Lavrov cautioned that Moscow could not "100 percent guarantee" that all the parties would obey the ceasefire.
"The Syrian Government has been informed by us about these arrangements, and it is ready to fulfil them," he added.
A truce agreed in February and endorsed by the United Nations Security Council has been repeatedly broken by both sides.
The final hours of the talks dragged out as Kerry contacted US President Barack Obama\’s office to get approval for the plan, but the top diplomat said both governments stand behind it.
"The United States is going the extra mile here because we believe that Russia and my colleague have the capability to press the Assad regime to stop this conflict and come to the table and make peace," Kerry said.
The UN\’s Syria envoy Staffan de Mistura described the deal as a "window of opportunity", and said he would discuss with UN Secretary General Ban Ki-moon when stalled political negotiations can restart.
German Foreign Minister Frank-Walter Steinmeier hailed the deal as "a real chance" to bring in urgently needed humanitarian aid, but warned that "everything now depends on rapid implementation".
Pro-regime forces have taken back a strategically important district on Aleppo\’s southern outskirts, rolling back nearly every gain from a month-long rebel offensive there.
The government advance further sealed off Aleppo\’s opposition-held eastern districts.
In another major blow to the rebels, the military commander of the Army of Conquest, the largest rebel alliance, was killed in an air strike, Islamist sources said Thursday.