The federal agency tasked with overseeing the U.S. nuclear stockpile has begun furloughing employees as part of the ongoing federal government shutdown, Energy Secretary Chris Wright said Monday.

In a visit to Nevada, Wright said the National Nuclear Security Administration is furloughing 1,400 federal workers as part of the shutdown, which began Oct. 1. Nearly 400 federal workers will remain on the job, along with thousands of NNSA contractors, the Energy Department said. The NNSA, a semi-autonomous branch of the Energy Department, also works to secure nuclear materials around the world.

“Tough day today,″ Wright said in Las Vegas before a scheduled visit to the Nevada National Security Site in Mercury, Nevada. ”We’re working hard to protect everyone’s jobs and keep our national stockpile secure,” Wright said.

The furloughs do not pose an immediate threat to national security, Wright said, adding: “We have emergency employees and the current nuclear stockpile is safe.”

President Donald Trump’s Republican administration fired hundreds of NNSA employees earlier this year, before reversing course amid criticism the action could jeopardize national security. Similar criticism emerged Monday after Wright’s announcement.

Wright said the disruption would affect employees and their families and will delay testing of commercial reactors, including some small modular reactors that the Trump administration has pushed as a cheaper alternative to costly nuclear plants that can take years or even decades to bring online.

“These are jobs of great gravity,” Wright said, urging congressional leaders to reopen the government as soon as possible.

Democratic Sen. Ed Markey of Massachusetts said it was “dangerously unacceptable that the Trump administration claims it will have to temporarily suspend certain nuclear security programs because of the ongoing government shutdown.”

“There is no justification for relaxing security and oversight when it comes to our nuclear stockpile,” Markey said.

House Armed Services Committee Chairman Mike Rogers, R-Ala., said lawmakers were informed of the pending furloughs late last week.

“These are not employees that you want to go home,” he said at a news conference Friday. “They’re managing and handling a very important strategic asset for us. They need to be at work and being paid.”

Republican Sen. Roger Wicker of Mississippi, who chairs the Senate Armed Services Committee, called the furloughs unacceptable.

“We cannot allow delays or interruptions to our nuclear programs during this shutdown. This is not a partisan issue, and for the sake of our national security” Congress should immediately reopen the government, Wicker said in a statement. “In the interim, it is incumbent upon Secretary Wright to work with Congress, OMB and the White House to ensure our nuclear weapons stockpile remains safe, secure and capable of deterring our adversaries.”

At the heart of the government shutdown are looming health insurance spikes for millions of people. Democrats are seeking negotiations on expiring health care subsidies while Republicans say they won’t discuss it, or any other policy, until the government reopens.

The February firings, which initially included NNSA workers, were part of a massive purge of federal workers led by then-Trump adviser Elon Musk and his Department of Government Efficiency.

One of the hardest-hit offices at the time was the Pantex Plant near Amarillo, Texas. Those employees work on reassembling warheads, among the most sensitive jobs across the nuclear weapons enterprise, with the highest levels of clearance.

Employees received furlough notices dated Sunday for 30 days or less, with an expiration date of Nov. 18. Employees who are not involved in performing critical functions such as those related to the safety of human life and the protection of property or working on the orderly suspension of operations were being placed in a furlough status without pay.

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Jennifer McDermott in Providence, R.I., and Ty ONeil in Las Vegas contributed to this story.

Latest federal workers to get furloughed: The people watching the nuclear stockpile