A drone missile strike killed a US Marine and a Navy corpsman last week by mistake, in what appeared to be the first instance US troops had been killed in a "friendly fire" incident involving an unmanned aircraft, defense officials said Monday.

The military has launched a probe into the incident, which appeared to stem from confusion on the battlefield in the southern province of Helmand, US officials told AFP.

Fighter jets and combat helicopters are usually called in to provide close air support for coalition troops pinned down by insurgent fire, while drones tend to be used for manhunts targeting Taliban figures.

The strike claimed the lives of Navy Seaman Benjamin Rast, 23, of Niles, Michigan, and Staff Sergeant Jeremy Smith, 26, of Arlington, Texas, officials said.

NBC News, which first reported the incident, said that the two service members were part of a unit ordered in to reinforce Marines coming under heavy fire from insurgents outside Sangin, the scene of fierce fighting for years.

The Marines near Sangin, watching a video feed from the armed Predator drone overhead, saw infrared images moving towards them and may have concluded those "hot spots" were insurgents instead of fellow Marines, NBC reported.

The investigation into last week's incident follows a detailed account in the Los Angeles Times of a February drone strike that went terribly wrong.

In the February strike, US operators of a robotic plane and special operations forces mistakenly thought a convoy of Afghan civilians in vehicles was a Taliban unit moving in.

The strike near a village in Oruzgan province killed 15-16 men, one woman and three children, the US military says, while Afghans believe 23 people died, including two children.