A bomb blast killed two American soldiers serving under Nato in southern Afghanistan, NATO'S International Security Assistance Force (ISAF) said on Friday.
The blast, which involved an improvised explosive device, occurred in southern Afghanistan on Thursday, ISAF said.
"The two service members killed were from the United States," the statement said without giving further details.
A British soldier also died on Thursday in an explosion in southern Afghanistan, the Ministry of Defence said.
Their deaths bring to 463 the number of international soldiers killed in Afghanistan this year, the deadliest of an eight-year anti-insurgency campaign being fought by about 100,000 Nato and US-led troops.
More than half of this year's dead have been from the United States, the biggest troop contributor.
Earlier Gordon Brown, the prime minister, warned that unless Afghanistan's government takes action against corruption he would not risk any more British lives there.
"I am not prepared to put the lives of British men and women in harm's way for a government that does not stand up against corruption," he said.
Britain has promised to send 500 more troops to Afghanistan – but Mr Brown said it was dependent upon progress in governance.
The prime minister said Hamid Karzai "needs a contract with the Afghan people; a contract against which Afghans, as well as the international community, can judge his success".
"International support depends on the scale of his ambition and the degree of his achievement in five key areas: security, governance, reconciliation, economic development, and engagement with Afghanistan's neighbours," Mr Brown added.
"If the government fails to meet these five tests, it will have not only failed its own people, it will have forfeited its right to international support."
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