A California federal judge on Thursday temporarily blocked the Trump administration from ordering the U.S. Department of Defense and other federal agencies to carry out the mass firings of thousands of recently hired employees.

U.S. District Judge William Alsup in San Francisco said during a hearing that the U.S. Office of Personnel Management lacked the power to order federal agencies to fire any workers, including probationary employees who typically have less than a year of experience.

Republican President Donald Trump and billionaire Elon Musk, who oversees the so-called Department of Government Efficiency, are spearheading an unprecedented effort to shrink the federal bureaucracy, including through job cuts.

Those efforts have resulted in a fierce pushback from Democrats, unions and federal workers, who argue the job cuts are illegal and could compromise government functions.

Already, the administration has been forced to recall some personnel in critical roles. But Trump has backed Musk to the hilt and has embraced Musk's goal of slicing $1 trillion from the nation's $6.7 trillion budget.

Budget experts say Musk, the CEO of SpaceX and Tesla (TSLA.O), opens new tab, is unlikely to reach his target by trimming jobs and reducing waste and fraud, and may have to slash government programs, including benefits.

On Thursday, hundreds of probationary workers at the National Oceanic and Atmospheric Administration, which conducts climate science, were notified they were being let go, according to a source familiar with the situation.

Officials at NOAA did not respond to a request for comment.

At the Internal Revenue Service, the head of the agency's Transformation and Strategy Office, a group of 60 employees working on modernization efforts, told his team Thursday there was a risk the whole office would be eliminated, according a person briefed on the matter.

David Padrino, chief of the office, told his team that he planned to resign effective a week from Friday, the person said, adding that IRS executives have been told to brace for a "drastic" cut to headcount in the coming weeks.

Meanwhile, OPM, the federal human resources agency, has instructed at least two dozen of its own employees working remotely that they must relocate to Washington in order to keep their jobs. They were given until March 7 to decide.

WIDESPREAD HARM

In his ruling, Alsup ordered OPM to rescind a January 20 memo and a February 14 email directing agencies to identify probationary employees who are not "mission-critical" and terminate them.

Alsup said he could not order the Defense Department itself, which is expected to fire 5,400 probationary employees on Friday, and other agencies not to terminate workers because they are not defendants in the lawsuit brought by several unions and nonprofit groups.

But he suggested that the mass firings of federal workers that began two weeks ago would cause widespread harm, including cuts to national parks, scientific research, and services for veterans.

"Probationary employees are the lifeblood of our government. They come in at a low level and work their way up. That's how we renew ourselves," said Alsup, an appointee of Democratic former President Bill Clinton.

The White House and the U.S. Department of Justice did not immediately respond to requests for comment.

The plaintiffs include the largest federal employee union, the American Federation of Government Employees, four other unions and nonprofits whose missions include advocating for services for veterans and conservation of national parks.

'I DON'T BELIEVE IT'

The Trump administration has maintained that the memo and email from OPM merely asked agencies to review their probationary workforces and decide who could potentially be terminated, and did not require them to do anything.

"An order is not usually phrased as a request," Justice Department lawyer Kelsey Helland told Alsup during the hearing.

But the judge said it was unlikely that virtually every federal agency independently decided to decimate its staff.

The judge specifically ordered OPM to communicate to the Defense Department by Friday that its memo and email regarding probationary employees are invalid. And it must give the same message to other agencies including the National Park Service and the Bureau of Land Management where staff cuts are likely to impact the nonprofits involved in the lawsuit, Alsup said.

The ruling will be in place temporarily while Alsup considers the legal challenge, which claims that OPM has no power over the hiring and firing of federal employees, and that its memo and email amounted to formal rules that can only be adopted through a lengthy administrative process.

Agencies began mass firings of probationary employees earlier this month. A second wave of mass layoffs targeting career employees began this week and a White House memo issued on Wednesday instructed agencies to submit plans by March 13 for a "significant reduction" in staffing.

Unions have filed several other lawsuits challenging Trump's efforts to reshape the federal workforce in the month since he took office, but have already faced procedural hurdles in pursuing them.