A civilian was among three Americans killed in Afghanistan in an incident still under investigation, NATO said on Saturday.
A brief statement from NATO's International Security Assistance Force (ISAF), released late Friday, said the deaths occurred in eastern Afghanistan earlier in the day.
"Two US servicemembers and one US employee were killed today in eastern Afghanistan," it said.
An ISAF spokesman confirmed to AFP the "employee" was an American civilian but said no further information was available on what happened or where.
Large parts of eastern Afghanistan border Pakistan's volatile tribal areas and have been the scene of fierce battles between Afghan and international forces fighting for the Kabul government against insurgents led from Pakistan.
The deaths of the US soldiers brings to 44 the total number of foreign troops to die in Afghanistan this year, according to the icasualties.org website, which shows 29 were Americans.
By comparison, the website says a total of 25 foreign troops were killed in January last year.
The United States and NATO have around 113,000 troops in the country, battling the Taliban, with another 40,000 being deployed through to August.
That extra deployment is being backed up by what US officials call a "civilian surge", as the renewed strategy for the war includes backing military action with aid and development.
Many of those civilians -- specialists in everything from wheat seeds to stress counselling -- are fanning out to military bases across Afghanistan, where they are vulnerable to Taliban attack.