US President Donald Trump has moved to cut nearly $5 billion of congressionally-approved foreign aid, the White House said Friday -- raising the likelihood of a federal shutdown as Democrats oppose the policy.

The $4.9 billion in cuts target programs of the Department of State and the United States Agency for International Development (USAID), Trump wrote in a letter to the House of Representatives.

The president "will always put AMERICA FIRST," the White House Office of Management and Budget said on social media, releasing a copy of the letter.

Democrats have warned that any attempt to reverse funding already approved by Congress would doom negotiations to avoid budgetary paralysis, the so-called shutdown, later this year.

Chuck Schumer, who leads the Democratic minority in the US Senate, described Trump's little-known legislative tactic, technically known as a pocket rescission, as illegal.

"It's clear neither Trump nor Congressional Republicans have any plan to avoid a painful and entirely unnecessary shutdown," he said.

Some moderate Republican also expressed opposition to Trump's effort to stop spending already approved by lawmakers.

A White House official told reporters the administration has a "solid legal basis" for Trump's maneuver -- and that any challenge in court would fail.

USAID dismantled

Trump has effectively dismantled USAID, the world's largest humanitarian aid agency, since taking office.

Founded in 1961 as John F. Kennedy sought to leverage aid to win over the developing world in the Cold War, USAID has been incorporated into the State Department after Secretary of State Marco Rubio slashed 85 percent of its programming.

Rubio welcomed Trump's latest move as part of "rooting out fraud, waste, and abuse from the US government, saving American workers billions of dollars."

The vast majority of the new cuts -- $3.2 billion -- would be to USAID funding, according to court documents seen by AFP, confirming an earlier report in the New York Post.

Research published in The Lancet journal in June estimated that the previous round of USAID cuts could result in the preventable deaths of more than 14 million vulnerable people worldwide -- a third of them small children.

Also targeted by the new cuts was $838 million for peacekeeping missions.

"This is going to make our budget situation or liquidity situation that much more challenging," United Nations spokesman Stephane Dujarric told a press conference.

Trump, after taking office for the second time in January, launched a sweeping campaign to downsize or dismantle swaths of the US government.

Republicans control both chambers of Congress, but need Democrat support in the Senate to pass new spending laws.

Trump, who is pushing to extend presidential powers, aims to claw back the $4.9 billion late in the fiscal year so that Congress may not have time to vote before the funding expires next month.

The United States last averted shutdown, with hours to spare, in March.

Shutdowns are rare but disruptive and costly, as everyday functions like food inspections halt, and parks, monuments and federal buildings shut up shop.

Up to 900,000 federal employees can be furloughed, while another million deemed essential -- from air traffic controllers to police -- work but forego pay until normal service resumes.

by Robin LEGRAND