Syria's army battled rebels and shelled neighbourhoods in Deir al-Zor on Saturday, killing at least 28 people in the eastern city in an oil-producing region close to the border with Iraq, opposition activists said.

The victims, who included three women and several children, were mostly civilians killed when shells hit their houses in the city's Old Airport and al-Hamidya districts, a source at a city hospital told Reuters.

"The death toll is likely higher. There are more bodies at the morgue, but they have not been identified yet," the source said.

Syria has restricted media access since the start of the uprising against President Bashar al-Assad in March 2011, making it hard to verify accounts from authorities or activists.

Loyalist forces have lost control of parts of the surrounding Deir al-Zor province, which borders Iraq's Sunni Muslim heartland as alliances between Assad's ruling elite and Sunni tribes have collapsed.

The artillery barrage on the Old Airport area on the edge of the city started late on Friday, following the defection of at least 30 members of the Hajjana, a border force that has a base in the area, opposition campaigners told Reuters from the city.

The central Al-Hamidya district came under shelling after Free Syrian Army rebels fought off a tank incursion into the area, they added.

Rebels have been mounting attacks on roadblocks, tanks and fortifications belonging to loyalist troops in Deir al-Zor, the provincial capital on the Euphrates river, 420 km northeast of Damascus.

A main oil pipeline from Deir al-Zor province feeds Syria's two refineries, in the city of Homs and an export terminal on the Mediterranean.-Reuters