Elon Musk, the billionaire advisor to Donald Trump, says all US federal employees must justify their work or lose their jobs, hours after the president pushed him to be “more aggressive” in slashing government spending.

Musk, the world’s richest person and Trump’s biggest donor, has led the effort to fire swaths of the government workforce.

However, the American Federation of Government Employees (AFGE), the largest federal employee union, vowed to challenge any unlawful terminations in a statement from national president Everett Kelley.

Kelley slammed Musk and the Trump administration, saying the move showed “their utter disdain for federal employees and the critical services they provide to the American people