A journalist inside Syria's second city, Aleppo, says rebels from the Free Syrian Army appear to be in control of about half the city.
However, the French journalist, Adrien Jaulmes, says the rebels expect a major assault by government forces with tanks trying to oust them.
Thousands of Syrian soldiers are being moved from the Turkish border to boost government forces in Aleppo. Restive neighbourhoods are already being pounded by artillery, mortars and helicopter gunfire.
Mr Jaulmes says the rebels plan to ambush and destroy as many government tanks as possible.
A BBC correspondent who has been in Aleppo says neither side can afford to lose the battle for the city. For the rebels, it would be a disastrous setback that, at the very least, could neuter their revolution for months. For President Bashar al-Assad, it could be the tipping point that presages the downfall of his government.
The correspondent says that despite their confidence and commitment, the rebels remain vastly outgunned and - with reinforcements from the army - outmanned, and it is hard to see how they can prevail.
Thousands of refugees continue to flee into neighbouring countries like Turkey and Lebanon.
More diplomats defect
Meanwhile, the United States has confirmed the defection of two more senior Syrian diplomats. Syria's representatives in the United Arab Emirates and Cyprus - who are husband and wife - are reported to have fled to Qatar.
White House spokesperson Jay Carney says the move shows that "senior officials around the Assad inner circle are fleeing the government because of the heinous actions taken by Assad against his own people, and the recognition that Assad's days are numbered".
The diplomats in question are Lamia Hariri, Syria's charge d'affaires in Cyprus, and her husband Abdel Latif al-Dabbagh, ambassador to the UAE.
A military attache at the Syrian embassy in Oman, Mohammed Tahseen al-Faqir, is also reported to have defected.