Some 1,400 workers at the National Nuclear Security Administration (NNSA) were due to receive notices telling them they had been placed on unpaid "furlough," CNN reported, leaving fewer than 400 at their posts.

It is the first time the agency has furloughed staff during a shutdown, and Energy Secretary Chris Wright was expected in Nevada to highlight the impact on the nuclear deterrent during a visit to the Nevada National Security Site.

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

The NNSA is responsible for designing, manufacturing, servicing and securing the weapons. It has fewer than 2,000 federal employees who oversee some 60,000 contractors.

The Department of Energy did not immediately respond to a request for comment.

At 20 days, America is enduring the longest full government shutdown ever -- the third-longest if partial shutdowns are included -- and President Donald Trump has been ratcheting up pressure on Democrats to vote to reopen the government.

Kevin Hassett, the director of the White House National Economic Council, told CNBC he expected the shutdown to end "some time this week."

But he warned Democrats that the administration "is going to have to look very closely... at stronger measures that we could take to bring them to the table."

Senate Republicans have offered a vote on renewing expiring health care subsidies for 24 million Americans -- Democrats' key condition for providing votes to approve a House-passed funding resolution that would reopen the federal bureaucracy.

But many Democrats insist that any deal Senate Majority Leader John Thune brokers will be meaningless without the sign-off of House Speaker Mike Johnson and Trump.

Johnson has vowed to keep the lower chamber of Congress closed until the shutdown ends, and it has already been out of session for more than a month.

The House already "did its job" and voted to fund the government, Johnson told ABC on Sunday.