State media said an explosion killed Defence Minister General Daoud Rajha, President Bashar al-Assad's brother-in-law, Assef Shawkat, and General Hassan Turkmani, head of the regime's crisis cell.

The FSA command "announces the good news of the outstanding operation this morning that targeted the National Security headquarters and the killing" of the officials "responsible for barbaric massacres," it said.

It made no reference to how the officials were killed, nor did a later one by someone identifying himself as a "field commander" of the FSA's Sahaba Brigades in Damascus.

First Lieutenant Ahmed Mohammed Taqa, in a video posted on YouTube, said "one of our heroes inside the place carried out a perfect security operation and killed them in a particular way about which I will not talk now."

He said six people had died.

To the three names officially announced, he added those of Interior Minister Mohammed al-Shaar, who was reported wounded; General Hisham al-Bikhtyar, head of the national security branch, and a Mohammed Saeed al-Khitan.

Officials said a suicide bomber struck as the ministers and security officials were meeting at the heavily guarded National Security headquarters.

The rebels said this operation was "the first in a series ... aimed at bringing down Assad and the pillars and symbols of the regime, whether civilian or military."

The FSA recalled an ultimatum it had already issued "to all those who do not have blood on their hands" to defect before the end of July, or else be deemed "accomplices with Assad in the murders."

Meanwhile, SITE Intelligence Group said another group, calling itself the Brigade of Islam, also claimed the attack in a statement on its Facebook page.

"The explosion came at the same time as a meeting of the senior criminals and the henchmen of the gang, and it led to the killing of several persons from amongst the pillars of the regime," said a message from the group.