Afghan troops were also wounded on Tuesday.
A US soldier was killed and two wounded in an operation in Afghanistan's Helmand province, where Afghan troops are battling to push back Taliban insurgents, a NATO defence official said Tuesday.
"We are deeply saddened by this loss," said Brigadier General Wilson A Shoffner, a spokesman for the NATO mission in Afghanistan, adding "our heartfelt sympathies go out to the families and friends of those involved".
Afghan troops were also wounded on Tuesday during the special forces mission near Marjah, a US defence official said.
NATO added the US troops were conducting a mission in support of their Afghan counterparts.
Afghan forces are currently fighting to repel Taliban insurgents who seized large swathes of the key opium-rich district of Sangin in the southern province of Helmand, a traditional stronghold of the insurgents.
The offensive prompted the first British deployment to the volatile province in 14 months.
The deployment, in addition to a recent arrival of US special forces in the region, comes a year after NATO forces formally ended their combat operations in the country.
A military helicopter that landed at the scene of the latest operation to carry out medical evacuations was grounded after a mortar exploded nearby. Crews were inspecting it for damage.
Colonel Michael Lawhorn, a spokesman for the NATO mission in Afghanistan, confirmed a US helicopter had landed in Marjah and was experiencing "mechanical problems".
"It was not shot down," he said.
A second defence official, again speaking on condition of anonymity, said the affected helicopter is a UH-60 Black Hawk.
A Taliban source claimed to AFP that the insurgents had shot the helicopter down, with all those on board killed.
The Taliban, who regularly exaggerate their battlefield claims, have in the past shot down several military helicopters with small-arms fire.
In October a US F-16 was struck by enemy fire in eastern Afghanistan, in a rare case of an advanced jet fighter coming under a Taliban-claimed attack.
In November the insurgents attacked a helicopter chartered by the Afghan army that crash landed in the north, killing at least three of those on board -- including a Moldovan crew member -- and taking others hostage.
Afghan officials said the helicopter had made an emergency landing due to "technical reasons", but the helicopter's private Moldovan operator Valan ICC said it had been shot down.
The unrest in Helmand, blighted by a huge opium harvest that helps fund the insurgency, comes after the Taliban briefly captured Kunduz city in September -- their biggest victory in 14 years of war.
US President Barack Obama in October announced that thousands of US troops would remain in Afghanistan past 2016, backpedalling on previous plans to reduce the force and acknowledging that Afghan forces are not ready to stand alone.