Shooting was reported in Baghdad Street, one of the city's main thoroughfares.

There were also reports of tanks in the south-western suburb of Midan, and clashes involving helicopters in the north and north-east of the city.

The fighting comes as UN-Arab League envoy Kofi Annan prepares to hold talks with Russian President Vladimir Putin about Syria.

Russia is a key ally of Syria and the meeting comes amid mounting pressure for increased international action against the country.

US Secretary of State Hillary Clinton said she hoped Mr Annan could persuade Russia to accept tougher measures.