Two members of the US military were accidentally killed last week in a drone attack, the first American victims of the unmanned aircraft, NBC has reported.

The two had been on foot and were approaching Helmand province's Sangin base, centre of some of the fiercest fighting in Afghanistan over the last decade. They were mistaken for the Taliban by marines, who were under fire at the time and who called in a missile strike from a Predator drone, NBC said.

The Pentagon is refusing to confirm or deny the story, saying that the incident is still under investigation. It reported last week that the two were killed but did not say where or how.

The two killed were named by the Pentagon as Marine Staff Sergeant Jeremy Smith, 26, from Texas and Navy Corpsman Benjamin Rast, 23, from Michigan.

The US has increasingly been using drones in both Afghanistan and Pakistan against suspected Taliban and al-Qaida targets, a controversial policy mainly because of the number of innocent civilians killed.

The marines under fire had been watching pictures of the battlefield being fed to them by the Predator. NBC said they saw a number of 'hotspots' – infra-red images – moving towards them and assumed, wrongly, they were the Taliban.

The hotspots had, in fact, been Smith and Rast, part of a unit sent to reinforce those under fire.