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.

Travis was convicted in September of indecently assaulting a woman and given a three month sentence suspended for two years.
A hearing at Southwark Crown Court heard that he had racked up a taxi fare bill of £4,456 during his trials.
He was awarded travel costs, limited to the value of a day return train ticket and £50 for a taxi, and hotel costs.
Stephen Vullo QC, representing Travis, made the defendant's costs application and said his client had been "financially devastated" by trials in February and September last year.
"He has no money left whatsoever," he said.
"He is now below zero."
Mr Vullo also claimed £630 in hotel accommodation, and £246 for travel to legal conferences, and asked for a total of £5,332 towards Travis's costs.
Judge Leonard ruled that Travis could claim the costs of a daily return train ticket from Aylesbury to Marylebone, which the judge said cost £61.40, as well as £50 a day for reasonable taxi costs from the station.
He allowed the hotel costs.
Referring to Travis's back problems, Judge Leonard said: "His infirmity was such that the benefit from the train, to get up from his seat rather than remain in it, is something that could only have been achieved on a train, not a car."
The defence estimated that Travis travelled from his home in Aylesbury, Buckinghamshire, to court on 37 days, which means he could be entitled to £4,121.80.
The number of days will be confirmed with the court at a later stage.
Prosecutor Joshua Munro argued the costs were not "reasonable" expenses for the taxpayer to fund.
Travis was cleared of 12 counts of indecent assault after a trial in February 2014.
The jury was unable to decide on verdicts on an additional count of indecent assault and one count of sexual assault and a retrial was ordered.
The Crown Prosecution Service added one more count of indecent assault, the offence of which he was eventually convicted, on the indictment for the September 2014 trial.
Travis was cleared of the indecent assault on which he was retried, and after a second jury was unable to reach a verdict on the sexual assault charge, Judge Anthony Leonard QC, directed a not guilty verdict.
One US service member has been killed and two wounded reportedly during a special forces mission in Afghanistan's southern Helmand province.
[NEXT_CONCEPT]
Former BBC Radio 1 DJ Dave Lee Travis was "financially ruined" by his lengthy legal battles, a court has heard.