The incident at the Pantex Plant in Amarillo, Texas occurred just before noon (1800 GMT), when a routine inspection identified "a potential concern with a vehicle," the National Nuclear Security Administration (NNSA) said in a statement.

"As a precaution, all employees were sheltered in place."

Police were called and authorities closed roads around the facility for about an hour while officials investigated.

"After searching the vehicle, it was determined there were no prohibited items or explosives, and the emergency event was resolved without incident," the NNSA said.

The plant is operated by government contractors and is the nation's primary one of six facilities for the assembly and dismantlement of nuclear weapons.

A recording on the plant's phone system said it was operating normally after what was initially described as a "security event."