London: Nearly 200 stealth fighters of the US Airforce, billed as world's most advanced warplanes built at a cost of billions of dollars, have been grounded because of problems with oxygen and power systems.

Test flights for F-22 Raptors have been suspended since May and production has been put on hold because of a suspected problem with oxygen, Daily Mail said.

The report quoted Air Force sources as saying that they have linked the fault with a fatal crash last November.
Last week, test flights for the newer F-35 Joint Strike Fighters, which are also made by Lockheed Martin, were also suspended because of a valve problem in the plane's integrated power system.
"It is third time this year the aircraft have been grounded and there is no indication as to when tests will resume," the report said adding that pilots have been told to return to their home stations until the jets are allowed to fly again.