The suicide bomber reportedly evaded security at the base and detonated an explosive belt in a room used as a fitness centre.
CIA chief's admission
In Washington, Leon Panetta, the CIA chief, said the seven killed "were far from home and close to the enemy, doing the hard work that must be done to protect our country from terrorism".
Among those killed was the chief of the CIA's operation at Forward Operating Base Chapman, the Associated Press news agency has learned.
Several other people, none of them US or Nato troops, were wounded in the explosion, US defence officials said.
"There has been a great deal of confusion when the reports emerged yesterday," Al Jazeera's Hashem Ahelbarra, reporting from Kabul on Thursday, said.
"We contacted the spokesperson of Isaf [the International Security Assistance Force] to confirm to us that US soldiers were killed.
"Then he came back to us in half an hour and said there had been a great deal of confusion and actually 'no, these are not US soldiers but civilians'. They are members of the PRT, which is the provincial reconstruction team."
The PRT was established in Afghanistan in 2002 by the US to assist in reconstruction efforts at district and provincial levels.
The US has committed to send hundreds of civilians to support work on development projects that aim to undermine support for the Taliban and other fighters.
But as the security situation in Afghanistan has deteriorated, many of the civilians working outside Kabul have retreated to army bases.
In Wednesday's second deadly incident, five Canadians were killed in the southern province of Kandahar.
The group, made up of four soldiers and a journalist accompanying them, were visiting community reconstruction projects and were killed when their armoured vehicle was hit by a bomb, the Canadian defence ministry said.
|Lang is the first Canadian journalist to die in Afghanistan since the mission began [AFP]
The journalist, Michelle Lang, worked for The Calgary Herald
The military has not disclosed the names of the Canadian soldiers because relatives have not all been notified.
Brigadier-General Daniel Menard, commander of multinational forces in Kandahar, said the soldiers were conducting a community security patrol
The Calgary Herald said Lang had been in the country since December 11 and was the first Canadian journalist to die in Afghanistan since Ottawa joined the international mission there in 2002.
The attack was the worst against Canada's military in the country in two years and brought its military deaths in Afghanistan to 138.
Canada has 2,800 troops in Afghanistan, but the mission has become increasingly unpopular at home and it is scheduled to be withdrawn at the end of 2011.