The United States has called on Syria to withdraw its forces from the flashpoint city of Hama, which is at the hub of an anti-regime revolt.

A spokesperson for the country's State Department said the Syrian authorities should 'allow Syrians to express their opinions freely so that a genuine transition to democracy can take place.'

The comments come in light of reports that at least 11 people in Hama have been killed by Syrian troops.

Tanks still surround the city, days after it witnessed some of the biggest protests against Bashar al-Assad's rule.

The attacks focused on two districts north of the Orontes River, which splits the city of 650,000 people in half.

Some residents of Hama, also the scene of a bloody crackdown by Assad's father nearly 30 years ago, had sought to halt any military advance, blocking roads between neighbourhoods with rubbish containers, burning tyres, wood and metal.

The advance by state forces took place a day after hundreds of troops and police entered the city in buses, carrying out raids in which at least three people were killed.

Hama has witnessed some of the biggest demonstrations and highest death tolls in Syria's 14-week uprising, inspired by revolts across the Muslim world.

‘Assad may wait to see whether large-scale protests in Hama continue. He knows that using military aggression against peaceful demonstrations in a symbolic place like Hama would lose him support even from Russia and China,’ Syrian activist Mohammad Abdallah told Reuters from exile in Washington.

The two countries have opposed a United Nations Security Council resolution proposed by the West against Syria, helping President Assad withstand mounting international isolation.

Mr Abdallah said using tanks to attack Hama would ‘totally discredit’ a promise by President Assad to seek dialogue with his opponents.

Troops and armour were already assaulting villages and towns in the Jabal-al-Zawya region, north of Hama, which had also seen large protests against President Assad's 11-year rule, he said.

The Syrian Observatory for Human Rights said tanks stormed the town of Kfar Nubbul early today ‘without meeting a single shot in the town that has seen peaceful protests since the beginning of the uprising.’