Swirling chaos in Syria may have prompted the mass exodus of up to as many as 30,000 Syrian refugees into Lebanon over the past 48 hours, according to UNHCR, the United Nations refugee agency.

The Syrian minister of defence Daoud Rajiha has been killed in a suicide bombing of the security headquarters in the capital of Damascus during a meeting between high-ranking officers and ministers.

The journalist Zaid Benjamin, who was the first to report on the rebels' Operation Volcano assault on the headquarters of President Bashar al-Assad's security aparatus, said that top officials were targeted. A barracks overlooking the presidential palace was said to be ablaze.

Republican guard troops had surrounded Shami hospital in Damascus after ambulances brought casualties from the explosion site.

"A suicide attack has targeted the National Security building," Syrian state TV said. "The terrorist explosion which targeted the national security building in Damascus occurred during a meeting of ministers and a number of heads of [security] agencies."

A number of Syrian officials were said to be "badly wounded", according to the BBC, which confirmed the death of Rajiha.

Fighting in the city between government forces and rebels has entered its fourth day. The violence is the most serious to hit Damascus since the uprising began 16 months ago.

The joint commander of the Free Syrian Army said a military operation dubbed "Damascus Volcano - Sham Cataclysm" had been launched to bring down the regime of Assad.

Syrian officials have denied claims of an all-out rebel offensive.