The US envoy for Syria has urged rebel groups to respect a shaky nationwide truce after they gave its brokers 48 hours to stop a regime offensive near the capital, AFP reports.

Twenty-nine rebel groups on Sunday set the deadline for the US and Russian sponsors of the February 27 ceasefire to halt an offensive by President Bashar al-Assad's regime in the Damascus region.

"We recognise that the CoH (Cessation of Hostilities) is under severe stress, but believe that to abandon it now would be strategic error," Michael Ratney said in a statement posted late Monday on Twitter.