The Pentagon has grounded its fleet of F-35 fighter jets after a routine inspection in California revealed a crack on the turbine blade of an engine.
Different versions are flown by the navy and the marine corps. All have been grounded.
The Pentagon says it is too early to assess the impact of the decision.
The fault was detected during a routine inspection of an air force version of the jet at Edwards Air Force Base in California, the Pentagon said.
The F-35 is the Pentagon's most expensive weapons programme with a cost of nearly $US400 billion.
It's the second time in less than two months that F-35 jets have been grounded due to technical problems.
The marine corps variant (F-35B), a short take-off and vertical landing variant (STOVL), was grounded for nearly a month after a manufacturing defect caused a fuel line to detach just before a training flight in January.
The air force version takes off from, and lands on, conventional runways while the STOVL version takes off from shorter runways and lands like a helicopter.