Hundreds of Syrian troops, backed by tanks, thrust into the eastern city of Deir Al Zor on Wednesday, opposition activists said, calling it a major offensive to root out rebels in the capital of the oil producing province on the border with Iraq.
Rebel sources said that in the last week fighters operating under the Free Syrian Army umbrella attacked roadblocks and tanks along the road from Deir Al Zor to the Iraqi border, killing or wounding dozens of troops, and destroying several tanks and armoured personnel carriers.
Large swathes of the province have fallen into rebel hands in the past few months.
Opposition campaigners said initial reports indicated army tank fire killed at least three people and wounded dozens in the neighbourhoods of Ommal, Jubaileh and Arfi in the city which lies on the Euphrates river, 430 km northeast of Damascus.
They added that Al Nour hospital in central Deir Al Zor was filling with the wounded.
Rebels fought back with rocket propelled grenades and hit four armoured vehicles. Saleh Al Shohat, a rebel commander, was killed by tank fire, they said.
"The army is trying to draw rebels from the villages and towns near Deir Al Zor into the city," Tayyeb Abu Al Kheir, one of the activists, said by satellite phone as the sound of tank fire echoed in the background.
"A government post office building was hit, to give you an indication of how random the bombardment is," another activist said.