More than 300 people were killed in violence across Syria on Thursday, making it the deadliest day of a 16-month uprising against President Bashar al-Assad's regime, a monitoring group said.

The Syrian Observatory for Human Rights said in a Friday statement that a total of 302 people -- 98 soldiers, 139 civilians and 65 rebels -- were killed across the country, revising an earlier toll.

"It's the highest toll since the beginning of the uprising, for civilians, rebels and soldiers," Rami Abdel Rahman, director of the London-based Observatory, told AFP.

He said the fighting was particularly fierce in Damascus and its surrounds, where 23 rebels and 47 civilians were killed in the sixth day of intense fighting in the capital between the army and rebel forces.

The Observatory had on Thursday put the toll at 248 people, including 109 civilians, but it warned that the number was expected to rise as fighting was ongoing and a number of casualties had not yet been identified.

The fighting, which has ratcheted up since Sunday, is the most intense since the anti-Assad revolt erupted in March 2011.

Heavy clashes broke out in Damascus on Sunday, with the rebel Free Syrian Army announcing "Operation Damascus Volcano" a day later.

On Wednesday, a bomb attack killed three senior members of Assad's regime, including his defence minister and the president's brother-in-law. The attack was claimed by the FSA.