The US Air Force (USAF) has grounded its entire fleet of F-22 fighter jets indefinitely after encountering a problem with the aircraft’s oxygen supply, USAF Air Combat Command (ACC) spokeswoman Captain Jennifer Ferrau has said.

Agence France Presse has quoted the official as saying, “We are still working to identify the exact nature of the problem. It is premature to definitively link the current issues to the OBOGS system.”

The radar-evading F-22 Raptors have been barred from flying above 25,000ft since January following the crash of a jet in Alaska during a training flight.

In another case, an aircraft scraped tree tops before landing leaving the pilot in a state of hypoxia, after which the planes were grounded.

Currently, the USAF has more than 160 F-22 Raptors in its fleet and has plans to build a total of 187 jets.