A US judge has temporarily halted President Donald Trump's plan offering incentives to federal workers to voluntarily resign before a Thursday midnight deadline.

Federal Judge George O'Toole Jr said the plan would be paused until a hearing on Monday when he could determine the merits of a lawsuit filed by federal employee unions, reported CBS, the BBC's US partner.

The offer is part of an ongoing effort by the Trump administration to slash the size of the federal government.

The White House says more than 40,000 employees have accepted the offer to resign in exchange for pay until 30 September. Some workers have voiced confusion about the terms of the deal.

The order came hours before Thursday's 23:59 EST (04:59 GMT Friday) deadline for federal workers to accept the offer.

A lawyer for the justice department said federal employees would be notified that the deadline had been paused, CBS reported.

The White House appeared to see the temporary halt as a way to increase the number of resignations.

"We are grateful to the Judge for extending the deadline so more federal workers who refuse to show up to the office can take the Administration up on this very generous, once-in-a-lifetime offer," press secretary Karoline Leavitt said in a statement.

An Office of Personnel Management (OPM) statement said the agency would continue processing resignations until an extended deadline of 23:59 local time on Monday.

"The program is NOT being blocked or canceled," it said. "The government will honor the deferred resignation offer."

The Trump administration, which previously said it hoped for as many as 200,000 employees to accept its offer, told US media they expected a spike in participation just ahead of the deadline.

"It's going to save the American people tens of millions of dollars," Leavitt told reporters outside the White House's West Wing before the judge paused the programme.

The American Federation of Government Employees, a union, filed the lawsuit against the OPM, arguing it had violated the law, that it could not fund the deal, and that it had given conflicting guidance about its terms.

The union said in an email to members that the offer was part of an "effort to dismantle the civil service and replace the skilled, professional workforce with unqualified political appointees and for-profit contractors".

The union noted that Congress has not passed a budget for funding beyond mid-March, arguing that it was unclear whether agencies could pay workers until September.

On Thursday, the union said it was "pleased" by the judge's actions.

Some federal employees have described their shock at the buy-out proposal, which was delivered in the form of a late-night email with the subject line "Fork in the Road". Some thought the email was spam.

"The tone of the initial email was like 'you may be cut anyway,'" Monet Hepp, a medical support specialist at the US Department of Veterans Affairs, previously told BBC. "People were blindsided by it."

Democrats have questioned the legality of the resignation package and warned it would lead to a "brain drain" that would be "felt by every American".

"Without the expertise and institutional knowledge that so many federal employees bring to their work, our government will be incapable of responding effectively to national emergencies, serving the American public, or even carrying out routine operations," Democrats on the House Oversight Committee wrote in a letter to President Trump.

On Tuesday, the Central Intelligence Agency became the first national security department to extend the offer to its staff. Former US intelligence officials and several lawmakers have raised concerns that this offer could undermine US national security.

There are also reports of impending cuts to the National Oceanic and Atmospheric Administration (NOAA), the weather-forecasting agency, and the Department of Health and Human Services.