Large numbers of Syrian troops are reported to be moving towards the country's second city, Aleppo, as part of efforts to recapture areas held by rebels.
The opposition Free Syrian Army says thousands of soldiers have been withdrawing from Idlib province near the Turkish border to redeploy in Aleppo.
Rebel fighters have taken over several quarters and the BBC reports that they are trying to advance.
Government forces have already been hitting back hard and reinforcements are reported to be on the way.
The escalation in Aleppo comes as two more ambassadors are reported to have defected.
The opposition Syrian National Council says the ambassadors to the United Arab Emirates and Cyprus, who are husband and wife, have defected to Qatar.
The ambassador to Iraq defected a fortnight ago.
Meanwhile, Turkey has barred its citizens from entering Syria and imposed restrictions on trucks carrying freight - a move correspondents say is tantamount to imposing economic sanctions on Syria.