Fierce fighting between Syrian government troops and rebels is taking place in a strategic district of the biggest city, Aleppo.
State media said government forces had taken control of Salah al-Din, but the Free Syrian Army later announced a successful counter-attack.
Images that have emerged from the northern city show buildings reduced to rubble by heavy weapons.
Observers say controlling Aleppo is a crucial goal for both sides.
In another development, Jordanian officials confirmed that former Syrian Prime Minister Riad Hijab crossed into Jordan only on Wednesday - two days after his defection was announced.
Activists said he and his family had been hiding in the south of the country and that reports on Monday that he had already left Syria were aimed at throwing government forces off the trail.
Source: BBC News