The US military has admitted it carried out secret race-based experiments during the Second World War to test the impact of mustard gas on its soldiers.

Puerto Rican and African American soldiers were tested separately in the belief that they might respond to certain chemicals in a different way to white troops, it has been reported.

As many as 60,000 troops were enrolled to test chemical agents including mustard gas in a programme that was declassified in the early 1990s.

The Pentagon has acknowledged that race may have been a factor in some mustard gas testing during the Second World War

But it is only now, after an investigation by National Public Radio, that the Pentagon has acknowledged that it grouped subjects for its tests by race.

African American Rollins Edwards, 93, revealed he was one of those subjected to experiments.

He told NPR that officers took him and 12 others into a wooden gas chamber before gas was piped in.

Mr Edwards, from Summerville, South Carolina, said: 'It felt like you were on fire. Guys started screaming and hollering and trying to break out. And then some of the guys fainted. And finally they opened the door and let us out, and the guys were just, they were in bad shape.'

Puerto Rican Juan Lopez Negron, now 95, told NPR he was also involved in the experiments - this time on a Panamanian island.

African American and Puerto Rican soldiers were tested separately during the Second World War in the belief that they might respond to certain chemicals in a different way to white troops (file picture)

Describing the effects of being exposed to mustard gas, he said: 'I spent three weeks in the hospital with a bad fever. Almost all of us got sick.'

The Independent, the Pentagon's press director, Army Col Steve Warren, confirmed NPR's findings - but looked to put distance between today's military and methods used during the Second World War.

In a statement carried by the Independent, he said: 'The first thing to be very clear about is that the Department of Defence does not conduct chemical weapons testing any longer.

'The US Department of Defence and Veterans Affairs Chemical Biological Warfare Exposure System is a database that includes 6,730 veterans possibly exposed to Mustard Agent or Lewisite.