Heavy clashes erupted for the first time in months in Syria's central city of Hama on Thursday as rebels tried to relieve pressure on comrades under attack from President Bashar al-Assad's forces elsewhere, activists said.

They said at least seven people were killed and dozens wounded when fighting broke out at 4 a.m. in Hama, a historic symbol of dissent against four decades of Assad family rule. Most of the reported casualties were civilian, they said.

Assad's father, who ruled Syria for 30 years until his death in 2000, crushed an armed Islamist revolt in Hama in 1982, killing many thousands of people and razing parts of the city.

Video uploaded by activists showed heavy machinegun and rocket fire amid cries of "God is great" from rebel fighters in Hama. Activists said it was the first time in six months that insurgents had engaged the army there with such ferocity.

"The operation is an attempt to alleviate some pressure on fighters in the countryside of Hama as well as in nearby provinces," an activist who called himself Safi al-Hamawi told Reuters via Skype. But he questioned the utility of the effort, raising concerns it may not be extensive enough to help.

Assad's forces have made gains on the Syrian-Lebanese border and in the suburbs of Damascus recently, as well as in parts of the north where rebels hold swathes of territory.

Hama, one of the few cities that had been relatively quiet, is close to Homs, near the Lebanese border. The province is an important link between the capital Damascus and pro-Assad Alawite strongholds on the Mediterranean coast.

Videos uploaded by activists showed rebels raising a black Islamist flag over a charred and bullet-scarred school they stormed on Thursday. They said it had been used as a military base and detention centre by soldiers and pro-Assad militias.

"This was one of the worst regime sites in the city. The soldiers treated residents harshly and in a sectarian manner," said an activist using the name Abu Adnan, speaking from Hama.

Hama, like Syria, is mainly Sunni Muslim but most loyalist militiamen and army officers are from Assad's Alawite minority, an offshoot of Shi'ite Islam.

At least five residential buildings were damaged in the fighting, according to Hamawi. "Residents are hiding in their homes, no one is fleeing. The fighting is still going on and very heavy, so people are too afraid to move."

Most shops in the city were shut, residents said by Skype. Telephone and mobile lines were later cut.

Opposition activists said the army had mobilised reinforcements and that it was not clear if the rebels could hold onto the captured school for long. – Reuters