Syrian government forces on Saturday launched a ground offensive to recapture rebel-held districts in the northern city of Aleppo, activists said, dpa reported.
Backed by fighter jets and helicopter gunships, soldiers loyal to President Bashar al-Assad clashed with rebels in southern and eastern districts of the city, they added.
"The regime forces are heavily shelling several areas and our revolutionary fighters are bravely confronting their attempt to storm (the southern district of) Salaheddine," Abu Omar al-Halabi, a commander in the rebel Free Syrian Army, told dpa by phone.
The opposition Syrian Observatory for Human Rights said fierce clashes were taking place in Salaheddine. It said three rebel fighters were killed.
Western countries have expressed concern for a massacre in Aleppo, a city of more than 2 million people near the border with Turkey.
Some 160 people were killed on Friday across Syria, including 17 in Aleppo, the observatory said.
Al-Assad sent reinforcements to Aleppo to recapture the city from rebels.
The outcome of the battle could determine the course of the 17-month revolt against al-Assad's regime.