The agency that oversees the US nuclear stockpile will temporarily lay off about 1,400 employees because of the ongoing shutdown, the US government shutdown now entering its 20th day.

Fewer than 400 employees will remain at the National Nuclear Security Administration (NNSA), a semi-autonomous agency of the US Department of Energy, the BBC reports.

NNSA’s Office of Nuclear Security, which is responsible for nuclear weapons transfers, has sufficient funding to be operational by 27 October.

The agency’s mission is to protect the U.S. nuclear stockpile and prevent nuclear weapons proliferation around the world.

It is the first time since its founding in 2000 that NNSA has laid off employees, with a spokesman saying “we have no choice this time.”

Thousands of federal employees have been laid off after a three-week government shutdown, with the White House warning of new cuts if the government does not reopen.

The government shutdown came after Republicans and Democrats failed to agree to pass a bill that would have funded government agencies in October and the coming months.

Democrats are pushing for increased health care funding in exchange for reopening the government. For their part, Republicans argue that health care issues should be addressed separately from budget negotiations when the government “reopens.”