The agreement calls for a nationwide ceasefire to begin on September 12th - the start of the Muslim Eid al Adha holiday.

If the cessation of hostilities holds for seven days, it will be followed by an unlikely military partnership between the US and Russia to target Islamic State and al Qaeda.

The Syrian government has told Moscow it is prepared to comply with the deal.

"Going after Nusra is not a concession to anybody," US Secretary of State John Kerry said.

"It is profoundly in the interests of the United States to target al Qaeda, to target al Qaeda's affiliate in Syria, which is Nusra, an organisation that is opposed to a peaceful transition, an organisation that is an enemy of the legitimate opposition, an organisation that is currently plotting attacks beyond Syria's borders, including against the United States."

Russian Foreign Minister Sergei Lavrov added: "I want to stress that distinguishing between terrorists and moderate oppositions has been written as a priority into the document that we agreed today." 

The Syrian government has told Moscow it is prepared to comply with the deal.

Opposition forces have also said they will fulfill the peace plan's requirements if the Syrian government demonstrates it is serious about ending the bloodshed.

Syria's moderate Free Syrian Army said they saw little chance of the deal's success, with a spokesman for one of its brigades saying that Damascus and Moscow had not observed the last agreement so were unlikely to observe this one.

"We hope this will be the beginning of the end of the civilians' ordeal," Bassma Kodmani, a leading member of the opposition umbrella group, the High Negotiations Committee, said. 

But Captain Abdul Salam Abdul Razak, a spokesman for the rebel Nour al-Din al Zinki Brigades, said the deal would only help the Syrian army to gather forces and send more Iranian-backed fighters into Aleppo.

'Turning point'

Truces agreed in the past have collapsed after both sides failed to hold up their ends of the bargain.

Mr Kerry has heralded the agreement, brokered in Geneva, as a potential "turning point" in the long-running civil war which has killed as many as 500,000 people and displaced millions of others.

However, he warned the plan could be described as nothing more than an opportunity until it is successfully enforced.

Mr Lavrov, who was also involved in the negotiations, said the deal would allow efficient co-operation in the struggle against terrorism, and expand humanitarian access to Syria's worst-hit towns and cities. 

Mr Kerry and Mr Lavrov have met four times since August 26th - and Moscow has been pressuring the Assad regime to suspend offensive operations against civilian areas and Syria's armed opposition.

Meanwhile, Washington was tasked with persuading "moderate" rebels to break away from the Nusra Front and other extremist groups.