The suicide bomber who killed eight people at a US base in eastern Afghanistan last week was a triple agent brought to the outpost after offering information to catch a leading al-Qaida aide.

The attack at Forward Operating Base Chapman in Khost province killed seven CIA employees and a Jordanian intelligence officer said to have brought the bomber, a Jordanian doctor, to the spy agency outpost.

US news agencies, citing intelligence sources, identified the attacker as Humam Khalil Abu Mulal al-Balawi, a 36-year-old doctor from the town of Zarqa, Jordan.

Balawi was arrested by Jordanian intelligence more than a year ago on suspicion of extremist sympathies, then apparently agreed to support the US in its fight against al-Qaida.

Jordanian authorities believed Balawi had reformed and handed him over to the CIA so that he could infiltrate al-Qaida in Afghanistan. He was invited to the remote base on the restive border with Pakistan after offering urgent information to help locate Ayman al-Zawahiri, Osama bin Laden's deputy, Associated Press reported. Officials said he was not searched for bombs when he entered the base. A CIA spokeswoman declined to comment.

The latest account of the attack contradicted a statement by the Taliban soon after the blast, which was said to have occurred as CIA officers began questioning Balawi at the base. A Taliban spokesman initially said the attacker was a sympathiser in the Afghan national army.

The CIA has vowed to avenge the attack, which killed four CIA officers, including the base's female chief, and three contract security guards.

Shortly after the attack, Barack Obama sent a letter of condolence to CIA employees, saying the spy agency has been tested "as never before" since the September 11 attacks. The letter, which was released to the White House press corps, was criticised for its open acknowledgement of the secretive CIA's role in the Afghanistan war.

The attack was the biggest loss of life for the CIA since the 1983 bombing of the agency's Beirut station, which killed 17 officers.

Zarqa, a bleak industrial town north-east of the Jordanian capital Amman, has spawned other killers. In 2006, a man from the town killed a British tourist and wounded five others when he opened fire on a tour group in Amman. Zarqa was also the hometown of Abu Musab al-Zarqawi, the self-styled leader of al-Qaida in Iraq who was killed by a US air strike in 2006.

The attack has occurred as the US is sending 30,000 more troops to the country, bringing the total number of its forces to about 100,000. Last year was the deadliest for US and Nato forces since the Taliban were overthrown by US-backed Afghan forces in late 2001. Bin Laden and Zawahiri, also a doctor, are said to have taken refuge in the mountainous border region between Afghanistan and Pakistan after the US invasion.