The soldiers were killed on Friday when an improvised explosive device (IED) detonated and damaged a tank manned by US forces in Farah province.
The NATO-led International Security Assistance Force in Afghanistan has rejected the claim.
A total of 418 US soldiers died in Afghanistan in 2011. The year 2010, however, remains the deadliest year for the US military with a death toll of 499.
Roadside bombs and improvised explosive devices (IEDs) are by far the most lethal weapon Taliban militants use against Afghan forces, foreign troops, and civilians.
The homemade explosives accounted for half of the about 1,500 civilian deaths in the first six months of 2011, according to the United Nations.
Insecurity continues to rise across Afghanistan despite the presence of about 130,000 US-led forces in the war-torn country