The Syrian army has turned its forces on Aleppo, ordering an armoured column to advance on the country's second biggest city.
Opposition activists have said the army is targeting rebels with artillery and attack helicopters.
As hostilities intensified near the Turkish border, Turkey said it is closing its crossing posts, although the UN said refugees fleeing Syria would be allowed through.
Reports from Syria cannot be independently verified due to state restrictions on foreign media.
The 16-month revolt against President Bashar al-Assad has been transformed from an insurgency in remote provinces into a battle for control of the two main cities, Aleppo and the capital, Damascus, where fighting exploded last week.
Mr Assad's forces have launched massive counter assaults in both cities.
They appear to have beaten rebels back from neighbourhoods in the capital and are turning towards Aleppo, a commercial hub in the north.
North of Aleppo, the town of Azaz has been almost completely destroyed by heavy fighting.
Burnt-out armoured personnel carriers sit on the roads where rebels hit them with rocket-propelled grenades. Bullet casings are scattered everywhere.
Fighting in and around Aleppo is expected to prompt an exodus across the Turkish border.
Some refugees are already complaining about poor conditions in Turkey and have clashed with riot police in disputes over food.
Further south, Syrian forces used artillery and fired rockets on the northern Damascus suburb of al-Tel in an attempt to seize it from rebels.
The attack forced hundreds of families to flee, residents and opposition activists said.
Elsewhere, Syria's ambassadors to the United Arab Emirates and Cyprus - a married couple - have deserted their posts, becoming the latest officials to abandon the Damascus government, rebels said.
