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Free Syrian Army units involved in a Russian-Turkish brokered ceasefire plan have demanded the regime ends its assault on an opposition enclave near Damascus or the rebel alliance will tear up the pact.

The armed opposition coalition issued an 8pm deadline to Damascus to end shelling of Wadi al-Badara valley, north-west of Damascus, or the FSA will respond with force.

"FSA factions involved in the truce agreement give Russia a notice until 8pm (Syria time) to stop the assault against Wadi Barada," said Osama Abu Zeid, legal representative for the Free Syrian Army on Twitter.

"Continuing the assault after 8pm will exempt factions from abiding to the truce agreement as Russia fails to commit. We call upon all opposition factions to raise military preparedness and join operations to save the Wadi Barada area."

Syrian opposition groups say the regime's allied foreign militias - notably Hizballah - are massing in the Ashrafiya area of the valley ready to launch a large-scale assault.

Barrel bombs and artillery have been dropped on the area, apparently softening up of rebel defences before a major offensive begins. The attacks have also killed or injured scores of civilians in the area, activists have said.

This puts under threat a Turkish-Russian brokered ceasefire signed between rebel groups and the Syrian regime earlier this week, which came into effect on 29 December evening.

Opposition groups say Wadi al-Barada has been relentlessly bombed and shelled since the ceasefire began and warned of an impending massacre if the assault doesn't stop.

Syria's regime blame the rebel groups in the area for allegedly poisoning the water supplies in the area which provides government-held Damascus with most of its water needs. 

Rebels deny the claims and say the regime bombed and damaged a water production facility, showing footage of a badly destroyed plant.

Opposition groups have also complained that a ceasefire agreement now being presented to the UN is different to an earlier copy they agreed to.