Eight Americans possibly working for the CIA were killed when a suicide bomber blew himself up after sneaking into the gym on a US base in Afghanistan, officials and US media reports said.

Four Canadian soldiers and a woman journalist were also killed when a bomb exploded as their armoured vehicle passed by on Tuesday, in one of the deadliest 24 hours for foreigners in the war-torn country.

The attacks come as the number of US and NATO-led foreign troops is set to soar to 150,000 to try to halt an increasingly virulent insurgency by the Taliban militia that has made 2009 the bloodiest year for international forces since the 2001 invasion.

Pentagon spokeswoman Lt Col Almarah Belk said the eight Americans died when an attacker detonated a vest packed with explosives on Forward Operating Base Chapman in Khost province -- a key Taliban stronghold.

"Eight Americans have been killed in an attack on RC-East," a US embassy official said, using the military term for a region of eastern Afghanistan.

A spokesman for NATO's International Security Assistance Force (ISAF) said that no US or ISAF military personnel were killed or injured in the bombing.

But the Washington Post newspaper said that most of the eight probably worked for the CIA, which it said was using the Chapman base. Related article: 8 killed in Afghan attack 'may be CIA'

A suicide bomber managed to penetrate the base's defences, detonating an explosive belt in a room described as a base gym.

Suicide attacks a hallmark

The Post said US sources confirmed that all the dead and injured were civilians, and that most were probably CIA employees or contractors.

It said the attack appears to have killed more US intelligence personnel than have died since the US-led invasion in 2001, adding that the agency has acknowledged the deaths of four CIA officers in Afghanistan since then.

Suicide attacks are a hallmark of the hardline Taliban militia, who are waging a major insurgency to topple the Western-backed government of Afghan President Hamid Karzai and oust the foreign troops.

The United States said last month it had doubled the number of civilian experts working in Afghanistan and was "on track" to meet its goal of nearly 1,000 by the new year. Many are to work in provincial military bases alongside military reconstruction teams.

The New York Times said an unidentified NATO official described Chapman as "not a regular base," suggesting it was used by US intelligence agencies.

The five Canadians were killed on Tuesday in a roadside bombing in the Taliban stronghold of Kandahar in southeastern Afghanistan, said General Daniel Menard, the head of Canadian forces in the country. Related article: Journalist among 5 Canadians killed

"Yesterday Canada lost five citizens," Menard said on Canadian television, adding that a Canadian civilian official was also wounded.

"Four soldiers and one journalist were killed as a result of an improvised explosive device attack on their armoured vehicle during a community patrol in Kandahar City."

Embroiled in controversy

Public television station CBC identified the journalist as Michelle Lang, a reporter with the Calgary Herald.

The deaths raised to 138 the number of Canadian soldiers killed in Afghanistan. Canada has some 2,800 troops deployed in the Kandahar region, who are supposed to return home in 2011.

The latest attacks came as international forces in Afghanistan -numbering 113,000 and set to grow to 150,000 next year - are embroiled in controversy over the deaths of Afghan civilians in an operation on Saturday.

In an incident that has inflamed public anger and strained ties between the Kabul administration and the Western military protecting it, President Karzai accused international forces of shooting dead 10 people, most members of the same family and eight of them teenagers.

The incident in eastern Kunar province sparked demonstrations on Wednesday, which saw protesters burning the US flag, shouting "Death to Obama" and calling for foreign troops to leave.

But ISAF said there was "no direct evidence to substantiate" the findings of investigators that unarmed civilians were killed.

- AFP