NATO’s International Security Assistance Force confirmed that troops had died following an improved explosive device attack, without naming their nationalities or giving further details.
But in Washington, an official said they were Americans.
NATO has some 130,000 troops in Afghanistan fighting an insurgency by Taliban Islamists who were ousted from power by a US-led invasion in 2001 for sheltering al Qaeda leader Osama bin Laden after the 9/11 attacks.
The Taliban are particularly active in the south and the east of the country, where NATO focuses most of its firepower ahead of its planned withdrawal from Afghanistan by the end of 2014.
Another NATO soldier died in a similar attack earlier Sunday, ISAF announced, taking the day’s toll to seven and the total toll for this year to at least 234, according to an AFP count based on the icasualties.org website.
As is traditional, ISAF deferred the identification of the casualties to their home countries.
Improvised bombs often made with fertilizeraccount for a large number of overall NATO casualties.
They also take a high toll among civilians who use the same roads, with 18 being killed in three separate attacks in the southern province of Kandahar earlier Sunday.
For the past five years the number of civilians killed in the war has risen steadily, reaching a record 3,021 in 2011 – the vast majority caused by insurgents, according to UN figures.