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(AFP) - The United States and Russia reached a deal on a new Syrian ceasefire Friday, which, if it holds, could see the first joint military effort by the two powers against Islamic jihadists.

US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov said the truce 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 of 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 Al Nusra jihadist rebels and Islamic State extremists.

- 'Window of opportunity' -

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 also killed large numbers of civilians.

In turn, Washington has to get the opposition groups it backs to separate themselves from Al Nusra, which has allied itself with a range of rebels at different points in the fluid conflict.

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.

- 'Back to square one' -
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, and regime troops backed by the Russian air force have completely encircled opposition-held neighbourhoods, leaving the civilian population completely cut off.

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.

"Rebels are now back to square one, under an even more ruthless siege," Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, told AFP.

In Aleppo, once Syria's economic powerhouse that has been ravaged by the conflict that began with anti-government protests in March 2011, desperate civilians described a battle for survival.

"This siege is much harder than the first one. During the first one, there were at least some products still in the market -- now there's nothing at all," said resident Omar al-Beik.

"No products, no vegetables, no sugar. Nothing. We came to buy a few things to cook and we couldn't find a thing," he told AFP.

In the nearby Al-Sakhur district, father of three Abu Omar said his family was surviving on rice, bulghur wheat and lentils, and had not had bread in three days.

"There's a risk that we'll be starving in two weeks," he said.

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