A judge has ordered several federal government agencies to reinstate the jobs of probationary employees fired en masse by the Trump administration last month.

Judge William Alsup called the sacking of these employees part of a "sham" strategy that aimed to circumvent proper procedures for reducing the federal workforce.

The order will apply to thousands of probationary workers who were fired at the Agriculture, Defence, Energy, Interior, Treasury, and Veterans Affairs departments.

During Thursday's hearing, the Department of Justice maintained that the firings were done based on guidance - rather than a directive - from the Office of Personnel Management (OPM).

The BBC has contacted OPM for comment.

OPM, a once-obscure agency that manages the federal government's civil service, has been thrust into the spotlight amid President Donald Trump's moves to slash the size of the federal workforce.

District Judge Alsup, appointed in San Francisco, countered the DOJ lawyer's arguments from the bench, citing evidence including termination letters that stated the firings were carried out on OPM's instructions.

"That should not have been done in our country," Judge Alsup said. "It was a sham in order to avoid statutory requirements."

Danielle Leonard, an attorney representing a coalition of government employee unions, said probationary employees were targeted because they lacked the right to appeal.

During the hearing, Judge Alsup also lamented the firing of a government worker in Albuquerque, New Mexico who had been given top marks for performance but was then issued a pink slip citing performance as the cause of their termination.

"I just want to say it is a sad day when our government would fire a good employee and say it's for performance when they know good and well that's a lie," Judge Alsup said.

White House press secretary Karoline Leavitt accused the judge of singlehandedly "attempting to unconstitutionally seize the power of hiring and firing from the executive branch".

She said that power rests with the president and "singular district court judges cannot abuse the power of the entire judiciary to thwart the president's agenda".

"The Trump administration will immediately fight back against this absurd and unconstitutional order."

Elon Musk's name was not mentioned during the hearing, but he has been tasked by President Trump with downsizing the federal workforce through the ad hoc Department of Government Efficiency - or Doge.

"He was on everybody's mind," said Luz Fuller, president of a local Sacramento branch of the American Federation of Government Employees, which represents more than 4,500 employees in Northern California.

The White House has denied that Musk is the agency's leader, although Trump labelled him as such during his Congressional address last week.