Russia and the US have announced an agreement on Syria starting with a “cessation of hostilities” from sunset on Monday.
Under the plan, the Syrian government will end combat missions in specified areas held by the opposition.
Russia and the US will establish a joint centre to combat so-called Islamic State and al-Nusra fighters.
The announcement follows talks between US Secretary of State John Kerry and his Russian counterpart, Sergei Lavrov.
The plan would need both the regime and opposition “to meet their obligations”, Mr Kerry said in Geneva.
The opposition had indicated it was prepared to comply with the plan, he said, provided the Syrian government “shows it is serious”.
Mr Lavrov said Russia had informed the Syrian government about the arrangements and the Syrian government was “ready to fulfil them”.
The accord also provides for humanitarian access.
“The cessation of hostilities requires access to all besieged and hard-to-reach areas, including Aleppo”, Mr Kerry said.
Seven days after the start of the cessation of hostilities, Russia and the US will establish a “joint implementation centre” to fight the Islamic State group and al-Qaeda-allied Nusra fighters.
Mr Lavrov said the joint implementation centre would allow Russian and US forces to “separate the terrorists from the moderate opposition.”
He said there would be co-ordinated strikes against the terrorists by the Russian and US air forces, and that in some areas, that excluded action by the Syrian air force.
“We have agreed on the areas where such co-ordinated strikes would be taking place, and in those areas, on neutral agreement shared by the Syrian government as well, only the Russian and US air force will be functional,” he said.
But Mr Lavrov added that “the Syrian air force will be functional in other areas, outside those that we have singled out for Russian-American military co-operation.”
A complex plan
Much depends on America’s ability to get opposition groups to distance themselves from their more extremist allies, and on Russia’s ability to restrain President Assad’s forces.
The Russian foreign minister Sergei Lavrov said Damascus had said it would fulfil its obligations.
But this is a complex plan and depends on all sides doing what has been agreed.
A deal in Geneva does not always become a reality on the ground.
Mr Lavrov and Mr Kerry stressed that the plan could pave the way for a political transition.
“The plan is more prescriptive and far reaching than any proposal to date and if implemented by all sides could allow political negotiations to take place on Syria’s future,” Mr Kerry said.
The United Nations envoy to Syria, Staffan de Mistura, welcomed the agreement and said the UN would exert all efforts to deliver humanitarian aid.
‘Apex of horror’
Fighting has recently escalated between Syria’s army and rebels in eastern Aleppo, where 250,000 people live.
On Sunday, Syrian government forces were reported to have recaptured parts of Aleppo which were lost to rebels last month, placing rebel-held districts in the city’s east once again under siege.
Staffan de Mistura warned earlier on Friday that food and water shortages made the situation in Aleppo even more serious than previously, and that fuel supplies could run out within days.
UN humanitarian chief Stephen O’Brien told the BBC conditions in Aleppo had become appalling: “Eastern Aleppo is at the apex of horror, where anyone of us if we were there would find life barely possible, let alone tolerable.”
The US and Russia support opposite sides in the conflict that began in 2011: Washington backs a coalition of rebel groups it describes as moderate, while Moscow is seen as a key ally of Syrian President Bashar al-Assad.