The agency responsible for safeguarding the US nuclear stockpile began placing most staff on enforced leave on Monday, US media reported, as the government shutdown dragged into a fourth week.

Some 1,400 workers at the National Nuclear Security Administration (NNSA) were expected to receive notices telling them they had been placed on unpaid “furlough”, CNN reported, leaving just 375 at their posts.

“Since its creation in 2000, NNSA has never before furloughed federal workers during funding lapses,” Energy Department spokesman Ben Dietderich told CNN.

“We are left with no choice this time. We’ve extended funding as long as we could.”

The United States has an arsenal of 5,177 nuclear warheads, with about 1,770 deployed, according to the global security charity Bulletin of the Atomic Scientists.

The NNSA is responsible for designing, manufacturing, servicing and securing the weapons, and oversees some 60,000 contractors.