BBC News with Stewart Macintosh

Syria state television has reported that three senior figures in President Assad's government have been killed in a suicide bomb attack in the capital Damascus. The president's brother-in-law, Assef Shawkat, was among the dead. He had been very influential in the security services. Also killed were the Defense Minister Daoud Rajiha and Gen Hassan Turkomani, who was heading the crisis management team that the bomber targeted. Frank Gardner reports.

In Damascus, the rumors are

rife

, but the conclusion is clear. The ruler President Assad is now under more serious threat than ever before. That does not mean it's

on the point of

immediate collapse, but the death of Assef Shawkat, the president's powerful brother-in-law, is a triple blow for the ruling family. He's the closest person to the president to be killed so far. The personal command and control he built up in the armed forces has now gone, and his apparent

assassination

in the heart of the capital will make others in the regime wonder if anywhere in the country is safe for them.

Syria's rebel Free Syrian Army has said it was responsible for the bomb attack in Damascus. The Syrian government said it was more determined than ever to hit back at what it called terrorists backed by the United States and Israel. But unlike previous incidents, Syrian state television has not shown images from the attack.

The United States says the killing of senior Syrian government figures shows that President Bashar al-Assad is