The US troops came under fire Tuesday in the town of Marjah while accompanying Afghan special operations forces, and a US helicopter was damaged in the incident, Pentagon spokesman Peter Cook said. Afghan troops also were injured in the fighting, Cook added.
American special operations troops were part of the mission accompanying Afghan forces, a US defense official said.
"US special operators are ... allowed to engage and train, advise and assist their special operations counterparts," Cook said. "They've been in Helmand province providing this kind of support in the past."
Two US HH-60 Pave Hawk medical evacuation helicopters were sent to provide assistance, Cook said. One was waved off after taking fire and returned to its base, while the second landed safely but struck a wall, damaged its rotor blades, and remained on the ground, Cook added, Reuters reported.
The American killed in the incident became the first US military death of 2016 in Afghanistan. The incident underscored the continuing danger faced by American troops who have been fighting in Afghanistan since 2001.
"My understanding is that there may still be Americans on the ground in this immediate situation engaging with the enemy in support of Afghan forces," Cook said. "This is a fluid situation."
For more than six months, Helmand has been the scene of battles between insurgents and security forces that have complained of being abandoned by the US-backed Afghan government. The deputy governor of the volatile southern province said in December that Helmand could fall to the Taliban after months of heavy fighting.
US Defense Secretary Ash Carter has been updated by US commanders in Kabul on the situation via videoconference, Cook said. White House spokesman Josh Earnest said President Barack Obama had been briefed on the incident.
Obama last year slowed the pace of the withdrawal of US troops from Afghanistan, announcing he would maintain a force of 9,800 through most of 2016.
The security situation in Afghanistan has worsened. A Pentagon report last month stated Taliban forces were able to stage more attacks and inflict more casualties on Afghan forces in 2015 compared to 2014.
"We believe we're on the right course," Cook said. "We remain confident in the future of the Afghan government and the Afghan security forces."