The statement by the Riyadh-based High Negotiations Committee came hours after Moscow and Washington said a ceasefire in war-torn Syria was set to begin overnight Friday-Saturday.

"After HNC general coordinator Riad Hijab presented the results of his discussions with opposition factions across all fronts we agreed to respond positively to international efforts to reach a truce deal" the HNC said.

But its "commitment to the truce is conditional" on the lifting of sieges release of prisoners a halt to bombardment of civilians and the delivery of humanitarian aid.

The statement said the ceasefire "should be announced simultaneously by all sides".

It quoted HNC head Hijab as saying he did not have high expectations that the regime and its allies "would be able to commit to stopping their hostile acts".

In a joint announcement earlier Monday the United States and Russia said parties in Syria that wanted to commit to the cessation of hostilities should do so by noon Damascus time (1000 GMT) on Friday.