The blast left another American and six Afghan soldiers wounded, the US-led NATO coalition said.
The news comes just a day after NATO announced the deployment of around 100 US troops to Lashkar Gah to help head off a potential Taliban takeover of Helmand’s capital as fighting intensifies.
“One US service member died as a result of wounds sustained during operations near Lashkar Gah in Helmand,” NATO said in a statement.
“The service member was killed conducting train, advise, assist activities with Afghan counterparts under NATO authorities when their patrol triggered an Improvised Explosive Device,” NATO added, without naming the soldier.
Fighting has recently escalated in the opium-growing province of Helmand, with insurgents coming within a few kilometres of Lashkar Gah — raising fears the city is at risk of falling.
“On behalf of all of US Forces – Afghanistan, as well as Resolute Support (NATO), our deepest sympathies go out to the families and friends of those involved,” said General John Nicholson, commander of US and coalition forces in Afghanistan.
“We are deeply saddened by this loss, but remain committed to helping our Afghan partners provide a brighter future for themselves and their children.”
The turmoil convulsing Helmand, blighted by a huge opium harvest that helps fund the insurgency, underscores a rapidly unravelling security situation in Afghanistan.
Fighting has left thousands of people displaced in Helmand in recent weeks, sparking a humanitarian crisis as officials report food and water shortages.
The Taliban have also closed in on Kunduz — the northern city which they briefly seized last year in their biggest military victory so far — leaving Afghan forces stretched on multiple fronts.
“This is a big effort by the Taliban. It’s probably the most serious push we’ve seen of the (fighting) season,” Brig Gen Charles Cleveland, spokesman for NATO forces in Afghanistan, said on Monday.
But coalition forces have insisted that neither Kunduz nor Lashkar Gah are at risk of falling to the insurgents.
NATO officially ended its combat mission in December 2014, but US forces were granted greater powers in June to strike at the insurgents as President Barack Obama vowed a more aggressive campaign.
The US intervention in Helmand has fuelled the perception that foreign powers are increasingly being drawn back into the conflict as Afghan forces struggle to rein in the Taliban.
The fighting in Helmand comes as Afghan troops are stretched on several other battlefronts across Afghanistan — including the eastern province of Nangarhar where the Islamic State group is making inroads.