US President George W Bush has vowed to hunt down those responsible for Monday night's deadly bombings in Saudi Arabia that US officials and terrorism experts agree were almost certainly masterminded by Al Qaeda.

FBI Director Robert Mueller said he was sending a team of agents to help Saudi authorities with the investigations.

At least 29 people were killed, seven of them Americans, when terrorists detonated three car bombs inside expatriate housing compounds in Riyadh.

"It says to me that they are alive and well. Some of the top management may have been captured and a lot of the troops may be dispersed, but Al Qaeda lives," said Jane Harman, a senior Democrat on the House of Representatives Intelligence Committee.

The Saudi ambassador to London blamed members of a group of 19 Al Qaeda suspects who evaded capture in Riyadh last week, some days after the United States warned that an attack against US interests was being planned.

The bombings were carried out just hours before a scheduled visit by US Secretary of State Colin Powell.

Sophisticated Operation

Assailants drove, guns blazing, into three guarded housing compounds for expatriates shortly before midnight and set off huge car bombs.

US defence and security officials described the operation as swift and sophisticated.

At one well-defended compound housing employees of US defence contractor, Northrop Grumman Vinnell Corp subsidiary, the bombers killed two Saudi soldiers and wounded two others at the main gate before blowing the front off a four-storey building housing unaccompanied or bachelor employees.

The company said seven US citizens and two Philippine nationals were killed.

US officials said the operation indicated the attackers had inside knowledge of the compound layout and knew which building had the most inhabitants, but many people could have had such knowledge.

They may not have known, however, that out of the 70 men who usually sleep in the building, 50 were out in the desert that night on an exercise with the Saudi military.