Military sources in Syria say there are 20,000 government troops have gathered in and around the northern city of Aleppo, amid fears that they are planning an all-out assault on the city.
Rebels say they are prepared for a fierce battle and will mount a strong offensive.
Meanwhile a pan-Arab television station has aired a video purporting to show Syrian rebels guarding a group of Iranians abducted a day earlier and promising more attacks on Iranian targets.
Armed men in the video identify themselves as members of the rebel "Baraa Brigades" and say that at least one of the 48 captives was an officer of Iran's powerful Revolutionary Guards.
They claim the Iranians were on a "reconnaissance mission" in the capital Damascus at the time they were abducted. Iran says they are pilgrims who were visiting a shrine.