This image made from video provided by Shaam News Network Tuesday, July 17, 2012, purports to show a Syrian tank in Damascus, Syria. (AP Photo/Shaam News Network via AP video) THE ASSOCIATED PRESS HAS NO WAY OF INDEPENDENTLY VERIFYING THE CONTENT, LOCATION OR DATE OF THIS PICTURE.

BEIRUT - 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. There was no immediate word on casualties.

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 metres (yards) from the U.S. Embassy.

State TV reported that the blast struck the building as Cabinet ministers and senior security officials were holding a meeting inside.

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 neighbourhood, causing "great losses among them." The Britain-based Syrian Observatory for Human Rights said army helicopters attacked the neighbourhoods of Qaboun and Barzeh.

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

But Russia remained at loggerheads with the U.S. and its European allies, and there appeared to be little hope that the U.N.'s most powerful body would unite behind a plan.

The key stumbling block is the Western demand for a resolution threatening non-military sanctions and tied to Chapter 7 of the United Nations Charter, which could eventually allow the use of force to end the conflict in Syria.

Russia is adamantly opposed to any mention of sanctions or Chapter 7. After Security Council consultations late Tuesday on a revised draft resolution pushed by Moscow, Russia's deputy U.N. ambassador Alexander Pankin said these remain "red lines."

Russia has said it will veto any Chapter 7 resolution, but council diplomats said there is still a possibility of last-minute negotiations.

News from © The Associated Press, 2012