The group, the Syrian Revolution Coordination Union, said 57 soldiers in Deir al-Zor, including two lieutenants and a captain, had defected to the demonstrators. It said residents had formed local committees and erected makeshift barriers to try to halt the advance of tanks and armoured vehicles inside the city.

"More tank columns are heading to Deir al-Zor. By using heavy weapons, security forces are waging war against their own people," the group said in a statement.

The official state news agency said: "Armed groups in Deir al-Zor cut off roads, terrorized citizens and attacked police."

It added: "An exchange of fire occurred. The police forces confronted these armed groups and are still chasing them... The inhabitants of Deir al-Zor have expressed their rejection of these actions which are bad for the homeland."

Syrian authorities have expelled most independent journalists since the uprising against Assad began in March, making it difficult to verify reports of fighting.

Reuters