Russia has called for an emergency UN Security Council meeting regarding a possible offensive in Syria's Idlib province.
Earlier, Russia warned that rebels might stage a chemical attack, and then blame it on the forces of President Bashar al-Assad.
The US has vowed a strong reaction to any chemical or biological attack.
Idlib is the last remaining opposition stronghold - and it's encircled by Syrian government forces.
Al Jazeera's Osama Bin Javaid reports:
