Washington, United States — Elon Musk, the world's richest person and President Donald Trump's controversial close advisor, said Monday the giant USAID humanitarian agency will be "shutting down" as part of his radical -- and critics say unconstitutional -- drive to shrink the US government.

Hours later, Secretary of State Marco Rubio indicated it was not disappearing but would come under his control.

"I'm the acting director of USAID," Rubio told reporters on a visit to El Salvador, accusing the agency of "insubordination."

Amid confusion over the future of the US Agency for International Development, employees were instructed by email not to go to their offices Monday. Some 600 staffers found themselves locked out of their computer systems, ABC News reported.

Around 50 demonstrators gathered outside the headquarters in downtown Washington, with signs including "Save USAID, save lives."

USAID is the aid arm of US foreign policy, funding health and emergency programs in around 120 countries, including the world's poorest regions.

It is also seen as an important source of soft power for the superpower in its struggle for influence with rivals including China.

Musk called USAID "a viper's nest of radical-left marxists who hate America" and said, "you've got to basically get rid of the whole thing."

The SpaceX and Tesla CEO -- who has massive contracts with the US government and was the biggest financial backer of Trump's campaign -- said he had personally cleared the unprecedented move with the president.

"I went over with him in detail, and he agreed that we should shut it down," Musk said in a discussion on his X online platform.

Unconstitutional?

Democrats, who hold the minority in Congress, are sounding alarm over what they say is an unconstitutional power grab by Trump and Musk.

Congress has authority over the US budget but Musk argues his so-called Department of Government Efficiency (DOGE) can decide how money is used.

Because Musk is neither a federal employee nor a government official, it remains unclear to whom he or his  agency are accountable -- other than to Trump.

The pace and intensity of Musk's operation, which is using employees brought from his own companies, has caught opponents off guard.

In one especially tense episode, Musk's team insisted on gaining access to the Treasury's highly sensitive payment system, which is used for dispatching trillions of dollars a year across the entire government. It also contains the personal data on swaths of Americans.

"I can think of no good reason why political operators who have demonstrated a blatant disregard for the law would need access to these sensitive, mission-critical systems," Democratic Senator Ron Wyden wrote in a letter to Trump's new Treasury secretary, Scott Bessent.

Trump responded to the criticism Monday, telling reporters Musk "can't do and won't do" anything without "our approval."