The High Negotiations Committee (HNC), which represents the main bloc of anti-government rebels, announced this morning that its co-operation was conditional on guarantees that Russia and other allies of the regime would stop air strikes and artillery bombardment on civilian areas.
Dr Riad Hijab, the alliance’s co-ordinator, said his group was committed to efforts to “end Syrian bloodshed and push all parties to the negotiating table”.
Representatives held an emergency meeting in Riyadh, Saudi Arabia, on Monday after the US and Russia negotiated the terms of a prospective truce to start on Saturday.
A temporary “cessation of hostilities” was initially planned to start last week following discussions in Munich but was derailed by continued fighting including a regime advance in Aleppo province.
The HNC said any further attacks by Assad and its allies would end the ceasefire and called resolutions from the UN and International Syria Support Group (ISSG) to be implemented to end sieges, allow the delivery of humanitarian aid, release detainees and stop bombardment.
Dr Hijab said he did not expect the Assad regime and its allies to stop its campaign, signalling that the HNC may be preparing to continue fighting.
In pictures: Russian air strikes in Syria
In pictures: Russian air strikes in Syria
Volunteers from Syria Civil Defence, also known as the White Helmets, help civilians after Russia carried out its first airstrikes in Syria
The aftermath of Russian airstrike in Talbiseh, Syria
Smoke billows from buildings in Talbiseh, in Homs province, western Syria, after airstrikes by Russian warplanes
Russian Air Forces carry out an air strike in the ISIS controlled Al-Raqqah Governorate. Russia's KAB-500s bombs completely destroy the Liwa al-Haqq command unit
Caspian Flotilla of the Russian Navy firing Kalibr cruise missiles against remote Isis targets in Syria
Caspian Flotilla of the Russian Navy firing Kalibr cruise missiles against remote Isis targets in Syria, a thousand kilometres away. The targets include ammunition factories, ammunition and fuel depots, command centres, and training camps
Russia claimed it hit eight Isis targets, including a "terrorist HQ and co-ordination centre" that was completely destroyed
A release from the Russian defence ministry purportedly showing targets in Syria being hit
A video grab taken from the footage made available on the Russian Defence Ministry's official website, purporting to show an airstrike in Syria
Russia launched air strikes in war-torn Syria, its first military engagement outside the former Soviet Union since the occupation of Afghanistan in 1979. Russian warplanes carried out strikes in three Syrian provinces along with regime aircraft as Putin seeks to steal US President Barack Obama's thunder by pushing a rival plan to defeat Isis militants in Syria
“Our position is clear,” he added. “We are doing our part in accordance with the ISSG agreement in Munich, and acting according to the limits of the authority granted to us by the components of the HNC.
“The HNC is going to continue to discuss, and consult with our international friends and allies, and then we will respond formally.”
Major questions remain over how the truce, which has not formally been accepted by the Syrian government, will be enforced and how groups will be reprimanded for any violations.
It was unclear how continued operations against Isis and other extremists by the US-led international coalition and Russia would affect the agreement.
The Kremlin has insisted the group is its main target but opposition groups and Western leaders have alleged its strikes have mainly targeted areas controlled by the opposition, killing countless civilians.
A five-page plan released by the US State Department set a deadline for compliance at midnight on 26 February and stipulated that there must be no attempts to seize territory, although the “proportionate use of force” for self-defence is allowed.
It was published after Barack Obama and Vladimir Putin agreed the plans in a phone call.
Josh Earnest, the official White House spokesperson, admitted it would be “difficult to implement”, adding: “We know there are a lot of obstacles, and there are sure to be some setbacks.”
The Russian President called the agreement a “last real chance to put an end to the many years of bloodshed and violence” and said in a televised address that while Moscow negotiates with the Assad regime, he expects Washington to do the same with the opposition groups it supports.
Ban Ki-moon, the UN Secretary-General also welcomed the agreement as “a long-awaited signal of hope to the Syrian people” but warned that much work lies ahead for its implementation.
Hours after the agreement was announced, Assad issued a decree setting parliamentary elections for 31 April, signalling possible co-operation with a UN Security Council resolution calling for a vote to be held during an 18-month transition period hoped to end the five-year conflict.
Elections were already due as the current parliament's four-year term expires in May.
Additional reporting by AP