In a setback for Donald Trump, a US judge on Thursday ordered six federal agencies to rehire thousands of probationary workers who were fired as part of the current administration's move to slash the size of the government, media reports said.

Judge William Alsup said citing "poor performance" for mass layoffs was "a sham in order to try to avoid statutory requirements," the New York Times reported.

Hearing a lawsuit brought by employee unions, Alsup ordered the departments of the Treasury, Veterans Affairs, Agriculture, Defense, Energy, and Interior to reinstate whoever had been fired improperly.

"It is a sad day when our government would fire some good employee and say it was based on performance when they know good and well that's a lie," Alsup was quoted as saying by the New York Times during a hearing at the US District Court in San Francisco.

After returning to the White House in January, Trump had ordered to fire thousands of federal employees as a part of his plan to cut spending programs.

His actions have faced several setbacks in courts with several judges issuing injunctions intended to halt them.