The Chapman airfield is reportedly used for launching drone planes
Eight Americans working for the CIA have died in a bomb attack in Afghanistan, the worst against US intelligence officials since 1983.
A bomber wearing an explosive vest entered Forward Operating Base Chapman in Khost Province, near Pakistan.
A Taliban spokesman said a member of the group working for the Afghan army had carried out the attack.
It has raised questions about the coalition's ability to protect itself against infiltrators, analysts say.
The bombing was one of at least three deadly incidents across Afghanistan on Thursday. Elsewhere:
- Taliban militants beheaded six men they suspected of being spies for the government in the southern province of Uruzgan, police said
- Four Canadian soldiers and a journalist died in a roadside bomb attack in Kandahar, in the most deadly attack on Canadians in the country for more than two years
- Two French journalists were kidnapped in Kapisa province, north-east of Kabul, along with their Afghan driver and interpreter, reports say
Taliban spokesman Zabiullah Mujahid told the BBC the Khost bomber was wearing an army uniform when he managed to breach security at the base, detonating his explosives belt in the gym.
Peter Greste, Kabul
This is not the first time an Afghan security official has turned on coalition forces.
Last March, an Afghan soldier shot dead two US troops, wounded a third and then turned the gun on himself. And in November, a policeman killed five British soldiers in Helmand.
Those breaches of security will raise new questions about the capacity of the Afghan authorities to screen recruits as they struggle to increase numbers and take over the war from coalition forces.
At the same time, the coalition is dramatically ramping up numbers of both military and civilian personnel in Afghanistan, in line with President Obama's surge.
This latest attack exposes the risks inherent in both elements of that strategy.
Unnamed US officials were quoted as saying that most if not all of the dead Americans were either CIA agents or contractors, although this has not been officially confirmed by either the CIA or the Pentagon.
A further six Americans are reported to have been wounded.
The death-toll was the worst suffered by the CIA since eight officers were killed in a 1983 attack on the US embassy in Beirut.
Reports say the Chapman base is used by provincial reconstruction teams - which include soldiers and civilians - and is protected by some 200 Afghan soldiers.
The base has been described as "not regular" - a phrase that implies it was a centre of CIA operations in Khost province, the BBC's Peter Greste in Kabul says.
It is the biggest single reported loss of life for the CIA since the war began in Afghanistan eight years ago, and the biggest loss for the US since October.
"We mourn the loss of life in this attack, and are withholding further details pending notification of next of kin," US state department spokesman Ian Kelly said.
A spokesman for Isaf, the international Nato force in Afghanistan, said that "no US and no Isaf military personnel were killed or injured" in the incident.
Khost province - which is one of the Taliban's strongholds - has been targeted by militants in the past year.
The number of foreign civilians deployed in Afghanistan has been rising as international efforts there focus increasingly on development and aid.
Civilians work alongside military reconstruction teams at provincial bases around the country.
A "civilian surge" was one of the three core elements of the new US strategy for Afghanistan announced by US President Barack Obama at the beginning of the month.
This has been the deadliest year for foreign troops since the 2001 invasion.