Syrian government forces are reported to be preparing for a major offensive on rebel-held parts of the northern city of Aleppo and its surroundings.
A Syrian security source said it would start "in the coming hours or days".
Opposition activists in the city told the BBC that military reinforcements, including Hezbollah fighters, had been sent to parts of Aleppo.
Government forces backed by Hezbollah, the Lebanese Shia Islamist group, last week recaptured the town of Qusair.
On Sunday, they retook the last remaining rebel-held villages in the strategically important area, which lies between the Lebanese border and the central city of Homs.