BEIRUT: Syria's state-run TV says the country's defence minister as well as Syrian President Bashar al-Assad's brother-in-law and deputy defence minister Assef Shawkat  were killed in a suicide bomb attack targeting a meeting of top security chiefs in Damascus on Wednesday, state television said.

A suicide bomber struck the National Security building in Damascus Wednesday during a meeting of Cabinet ministers and senior security officials, state-run media said.

The blast came as the capital has seen four straight days of clashes pitting government troops against rebels, who are trying to bring down the regime by force.

The fighting is an unprecedented challenge to government rule in President Bashar Assad's seat of power and a sign the civil war is likely to worsen as the Syrian regime struggles to halt the opposition's growing momentum.

The violence has grown increasingly chaotic over the course of the uprising that began last year. Besides the government crackdown, rebel fighters are launching increasingly deadly attacks on regime targets, and several massive suicide attacks this year suggest al-Qaida or other extremists are joining the fray. Activists say more than 17,000 people have been killed in the uprising.

The state-run news agency SANA reported that Wednesday's blast was aimed at the National Security building, a headquarters for one of Syria's intelligence branches and less than 500 meters (yards) from the US Embassy.

Witnesses said police had cordoned off the area, and journalists were banned from approaching the site.

Earlier Wednesday, SANA said soldiers were chasing rebels in the Midan neighborhood, causing ''great losses among them.'' The Britain-based Syrian Observatory for Human Rights said army helicopters attacked the neighborhoods of Qaboun and Barzeh.

A UN vote was set for later Wednesday on a new Syria resolution.