Afghan forces are currently fighting to repel Taliban insurgents who seized large swathes of the key opium-rich district of Sangin.
A US soldier was killed and two wounded in an ongoing battle in Afghanistan's Helmand province, where Afghan troops are pushing back Taliban insurgents, the Pentagon said.
The troops had come under fire on Tuesday while conducting a mission with Afghan special forces in Marjah, Pentagon press secretary Peter Cook said.
"This is an ongoing situation, there is still a fight going on in the immediate surroundings," he said.
Two HH-60 Pave Hawk medical evacuation helicopters were scrambled after the attack.
One of these turned back after taking fire, and returned safely to its base.
The second landed at the scene but its rotor blades were damaged after it apparently struck a wall, Cook said.
Initial reports were that a mortar had exploded near the helicopter.
"There are dangerous parts of Afghanistan where the fight is still under way, and Helmand province is one of those places," Cook said.
"This is an ongoing fight, and I think the events of the last few hours highlight that."
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 Taliban 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 Taliban source told AFP that the insurgents had shot the helicopter down, with all those on board killed.
The Taliban, who regularly exaggerate 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.
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.
An unspecified number of Afghan troops were also wounded in the mission, officials said.
"I think the events of the last few hours in Afghanistan highlight the risks that the Afghan forces are taking every single day, and of course, the risks that the American forces who are there assisting them are taking as well," Cook said.