Syrian Defence Minister Daoud Rajiha has been killed in a suicide bombing at the headquarters of the National Security Bureau in the capital Damascus, Syrian state TV says.

Others are said to be critically hurt. Ministers and officials were meeting inside the building at the time.

The area around the building, in Rawda district, has been sealed off.

The attack comes amid claims of a major rebel offensive on the city. Syrian officials have downplayed the attacks.

Meanwhile, UN chiefs have been trying to persuade China and Russia to agree tougher measures on Syria, ahead of a Security Council vote on Wednesday on imposing sanctions.

UN-Arab League envoy Kofi Annan held talks with Russian President Vladimir Putin in Moscow, and UN Secretary General Ban Ki-moon met Chinese leader Hu Jintao in Beijing.

Footage purporting to shows a barracks overlooking the presidential palace on fire was posted on the internet

The UN has until Friday to renew the mandate for observers in Syria, although a vote is expected in New York on Wednesday afternoon.

Western nations want a new resolution threatening measures short of the use of force.

The Western-backed draft resolution to be discussed gives the Syrian government 10 days to withdraw heavy weapons from cities and return troops to barracks, otherwise a further resolution on sanctions will be submitted to the Security Council.

But the BBC's Jim Muir in neighbouring Lebanon says that with Russia resisting all efforts to persuade it to take a tougher line with Syria, there is virtually no hope of concerted international action to pull the country back from the brink.