Seven soldiers killed in a bombing in southern Afghanistan on Thursday were Americans, the Pentagon confirmed, adding that they died after successive blasts struck them at the same location.
Pentagon spokesman Colonel Dave Lapan said the soldiers were killed by two blasts in Shorabak district in Kandahar province. The attack was confirmed by the NATO-led International Security Assistance Force (ISAF).
Kandahar is the spiritual heartland of the Taliban and the intensity of fighting there in the coming months will be a key test of ISAF's ability to hold ground in the south taken from insurgents last year after a troop surge.
Local border police commander Tafseer Khan Khogyani said the attack also killed two Afghan policemen. It happened as coalition and Afghan forces were on patrol around 20 kilometres from Afghanistan's border with Pakistan, he added.
"As they approached a container, explosives that had been placed inside went off, causing a huge explosion," he said.
Responsibility for the blast was claimed by a spokesman for the Taliban.
The bombing caused ISAF's highest death toll in a single incident since April 27, when nine Americans -- eight troops and a contractor -- were killed by an Afghan officer who opened fire at a Kabul military training centre.
At least 199 foreign troops have now been killed in Afghanistan this year, according to an AFP tally based on that kept by the independent website iCasualties.org.
Earlier Thursday, a NATO helicopter crashed in a mountainous area of eastern Afghanistan, killing one soldier.
There are around 130,000 ISAF service personnel in the war-torn country, around 90,000 of whom are from the United States.
Much of Afghanistan's worst fighting takes place in the south of the country, particularly in the provinces of Kandahar and Helmand which border Pakistani areas where insurgents have hideouts.
While international forces insist they have been taking the fight to insurgents, the Taliban announced the start of their spring fighting season at the end of April.
The commander of foreign forces in Afghanistan, US General David Petraeus, warned in a memo released Saturday that they could face tough times ahead.
"It is likely that our enemies will pursue high-profile attacks this summer in an attempt to demonstrate continued capability," he said.
This should be expected because of the "progress" made in "important areas" since last year, he added.
There has been a rash of insurgent attacks against forces loyal to President Hamid Karzai's government in recent days, including a suicide attack on a Kabul military hospital Saturday which killed six medical students.
It is nearly 10 years since US-led forces invaded Afghanistan after the September 11, 2001 attacks to topple the Taliban, who had been harbouring Al-Qaeda leader Osama bin Laden -- killed by US forces in Pakistan this month.