The forces were taking part in a counter-terrorism operation when they came under fire, US officials said.
A medical helicopter sent to rescue casualties was forced to land and has yet to be recovered.
Some 12,000 international soldiers are still deployed in Afghanistan to help local forces tackle the Taliban.
Some of the foreign troops were sent to counter the recent militant insurgency in Helmand.
Details of what happened in the incident, which took place near the town of Marjah on Tuesday, remain sketchy.
US military spokesman Brig Gen Wilson Shoffner said one service member had died as a result of wounds, adding: "We are deeply saddened by this loss... our heartfelt sympathies go out to the families and friends of those involved."
The identity of the casualties has not yet been revealed and no official details given on their service branch.
A number of Afghan forces were also reported to have been wounded.
The casualties occurred before the "medevac" helicopter was called in.
Some reports said the helicopter, believed to be a UH-60 Black Hawk, had also come under fire.
A US defence official said it had "sustained damage while on the ground" but had not been shot down.
He said it remained on the ground and the military was working to recover it, but gave no further details.
In 2011, 30 Americans were among 38 people killed in a Chinook helicopter crash in Wardak province.
Helmand has long been a stronghold of the Taliban.
Afghan forces, aided by the US military, have been engaged in a fierce conflict recently with Taliban fighters who seized large swathes of Sangin district.
The fighting also sparked the deployment of a number of UK forces to Helmand for the first time in more than a year.
In September, the Taliban briefly overran the northern city of Kunduz in one of their biggest victories in 14 years of war.
One US service member has been killed and two wounded reportedly during a special forces mission in Afghanistan's southern Helmand province.