Syria Defense Minister Gen. Dawoud Rajiha reportedly killed in explosion.

(CBS News) LONDON - An explosion inside the Syrian national security headquarters in Damascus targeted ministers from President Bashar Assad's regime who were meeting with defense officials on Wednesday, reportedly leaving some senior officials dead and wounded.

Syrian TV reported that Syrian Defense Minister Gen. Dawoud Rajiha was killed in the explosion. His death could not be independently confirmed, but State TV is a mouthpiece for the Assad regime.

State TV reported that Assef Shawkat, Sryia's Deputy Chief of Staff and Assad's own brother-in-law, was also killed in the explosion. Other ministers and military officials were seriously injured in the explosion, according to the state-run channel.

Syrian State TV denied another report that Interior Minister Mohammed al-Shaar was dead, saying he was hospitalized in stable condition.

Opposition leader Kamal al-Labwani told al-Arabiya, a pan-Arab satellite channel, that a member of the rebel Free Syrian Army planted a TNT and C-4-based device inside the building ahead of the meeting, and then detonated the bomb remotely before fleeing to safety. There were conflicting claims, however, from other opposition figures, some of them claiming a suicide belt had been used in the attack.

CBS News' George Baghdadi reports from the scene of the explosion that the blast did not appear to be very large. There have been previous bomb attacks inside Syria targeting regime officials, but they have been much larger explosions and targeting much lower-ranking members of the government and security forces.

The national security headquarters is among the most tightly secured buildings in Damascus, suggesting the individual who planted the device likely was an insider. Officials told Al Jazeera that the person who planted the device was one of the cabinet member's personal bodyguards.

The armed uprising against Assad's regime has intensified rapidly in recent weeks, with mounting military and political defections eroding his family's long-time grip on power as rebel forces engage his troops more frequently in Damascus, the president's seat of power.

The Free Syrian Army announced in a statement on Monday the launch of an offensive dubbed "the Damascus volcano and earthquakes of Syria," reports CBS News' Khaled Wassef.

The statement said the FSA had begun carrying out attacks on all security stations and branches in Syrian cities and the countryside. It called for attacks on military forces and pro-regime militia checkpoints across the country, as well as cutting off all international highways.

The Muslim Brotherhood, a political party which has risen to new heights in nearby Egypt since the overthrow of President Hosni Mubarak, is the main faction in the biggest Syrian opposition group, the Syrian National Council. The Brotherhood issued a statement Tuesday urging the Syrian people to rise up and back the rebels locked in what the group called a "decisive battle" against Assad's troops in Damascus.