The suicide bomber reportedly evaded security at the base and detonated an explosive belt in a room used as a fitness centre on Wednesday.
A former senior CIA officer who was stationed at the base said a combination of agency officers and contractors operated out of the remote outpost with the military and other agencies.
Initial reports suggested the men killed had been soldiers.
"There has been a great deal of confusion when the reports emerged yesterday," Hashem Ahelbarra, Al Jazeera's correspondent reporting from the Afghan capital, Kabul, 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.
US media reports said the Americans killed were employed by the CIA.
The Washington Post newspaper, citing US officials, said the eight killed were working for the CIA, while the Associated Press cited an unnamed US official as saying CIA employees were believed to be among the dead.
According to The Washington Post report, the CIA has been bolstering its ranks in Afghanistan in recent weeks, mirroring the increase in troops.
The CIA has not yet commented or confirmed the deaths.
The base in Khost, known as Forward Operating Base Chapman, is a centre for personnel working on reconstruction projects in the country.
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.
Several other people, none of them US or Nato troops, were wounded in the explosion, US defence officials said.
The five Canadians were killed in a attack in the southern province of Kandahar just hours later.
The group, made up of four Canadian 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.
The journalist, Michelle Lang, was with The Calgary Herald.
The paper said Lang had been in the country since December 11 and was the first Canadian journalist to die in Afghanistan since Canada 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.