Syrian troops have killed 20 people in a tank assault on the eastern city of Deir al-Zor, an activist group said, despite a direct UN appeal to President Bashar al-Assad to stop using military force against civilians.

The assault on Deir al-Zor, capital of a restive oil-producing province, began a week after Assad sent the army to seize control of the city of Hama, a focal point of nearly five months of protest against his rule.

The Syrian Revolution Coordinating Union said most casualties in yesterday's attack on Deir al-Zor were in al-Joura district in the west of the city.

The military assault on Deir al-Zor, about 400km northeast of Damascus, was launched a day after UN Secretary General Ban Ki-moon told Assad he was alarmed by the escalating violence and demanded he rein in the army.

Ban "urged the president to stop the use of military force against civilians immediately".

Residents of Deir al-Zor, situated on the Euphrates River in a province bordering Iraq's Sunni heartland, had been bracing for an assault on their city.

A video posted on the internet last week showed a tribal meeting discussing armed resistance to any military move against them.

Several hours after the tanks moved in, explosions could be heard during telephone calls to residents in al-Joura.

Turkish Prime Minister Tayyip Erdogan, who had close ties with Assad before the uprising, said on Saturday his foreign minister would visit Damascus tomorrow to deliver a message to Syria. Turkey has grown increasingly critical of Assad. A senior minister last week called the Hama attack an atrocity.