WASHINGTON: A suicide attack in Afghanistan that killed seven agents marks one of the deadliest blows ever for the CIA, dealing a painful setback to an agency increasingly on the frontline of US wars.

The Central Intelligence Agency lowered the flag to half-mast at its tightly guarded headquarters in the Washington suburb of Langley but did not release the casualties’ names, who died cloaked in the same anonymity with which they lived.

“Your triumphs and even your names may be unknown to your fellow Americans, but your service is deeply appreciated,” President Barack Obama wrote in a letter to CIA employees.

Obama said since the September 11, 2001 attacks on the United States, “the CIA has been tested as never before.” “Because of your service, plots have been disrupted, American lives have been saved and our allies and partners have been more secure,” Obama said.

He said stars would be added in their name to the 90 already on the Memorial Wall at CIA headquarters honouring spies who have fallen in the line of duty.

The CIA said a Taliban bomber managed to penetrate the defences of a forward base in Khost, a pivotal province near the Pakistan border, detonating an explosives belt in a room described as a gym.

“This attack is something that will never be forgotten in Langley, Virginia,” said Jack Rice, a former CIA officer in Afghanistan and talk-show host.

“The impact can be huge, not just in terms of the capabilities of these particular people, but in the relationships that they themselves have built,” he said.

“You can’t simply go pick up five or 10 more of these guys. They may be the best guys in the world at what they do and they’re gone,” he said.

Rice said it was possible that the base let its guard down in searching the bomber, who may have been coveted as an informant.

Seven CIA employees died and another six were injured, with their lives saved by US military doctors and nurses, CIA director Leon Panetta said.

It was the deadliest single incident for the CIA since 1983, when eight agency employees were believed to be among the dead when Islamic militants bombed the US Marine barracks in Beirut, killing 241 Americans and 58 French.