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 Taliban on Thursday claimed responsibility for the suicide attack in Khost, a spokesman told AFP.

"We claim responsibility for the attack," purported Taliban spokesman, Zabiullah Mujahid, told AFP by telephone from an undisclosed location.

The Taliban spokesman, however, claimed the attack killed 16 Americans.

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.