The Republican chairwoman of the Senate Appropriations Committee is joining Democrats in slamming President Trump’s use of a rare tactic to withhold almost $5 billion in foreign aid funding that was already approved by Congress.

The move is also causing consternation at the United Nations, which stands to lose a substantial portion of its funding that was included in the appropriation. UN officials were also grappling with an unrelated move by the Trump administration to withdraw visas from the Palestinian delegation to the world body.

Mr. Trump announced in a letter to the House speaker, Mike Johnson, late Thursday that he will not spend $4.9 billion in congressionally approved foreign aid. The letter was posted Friday morning on the X account of the White House Office of Management and Budget.

According to the Associated Press, Mr. Trump is employing so-called pocket recissions in which the president announces a plan to withhold approved funds too late in the budget year — which ends September 30 — for Congress to respond. The legally questionable tactic has not been employed in almost 50 years.

“The proposed recissions affect programs of the Department of State, as well as the United States Agency for International Development and International Development Programs,” Mr. Trump’s letter says.

The Republican chairwoman of the Senate Appropriations Committee, Susan Collins, denounced the action Friday, charging in a public statement that “any effort to rescind appropriated funds without congressional approval is a clear violation of the law.”

“Instead of this attempt to undermine the law, the appropriate way is to identify ways to reduce excessive spending through the bipartisan, annual appropriations process,” she said.

Leading Democrats, for their part, said the move sabotaged the chances of any bipartisan budget agreement ahead of a September 30 deadline. Any resolution to fund the government beyond that date will require at least some Democratic votes in the Senate, where 60 votes will be needed.

“Today’s announcement of the administration’s plan to advance an unlawful ‘pocket rescission’ package is further proof President Trump and congressional Republicans are hellbent on rejecting bipartisanship and ‘going it alone’ this fall,” the Senate minority leader, Chuck Schumer, said.

Apart from more than $3 billion for international development assistance and a smaller amount for democracy building around the world, the action withdraws $520 million for the United Nations’s regular budget and its organizations such as Unesco, as well as $838 million for international peacekeeping operations, the Associated Press reported.

The UN spokesman, Stephane Dujarric, said the world body will follow up with American authorities to get more details on its plans. “What can I tell you?” the AP quoted Mr. Dujarric as saying. “This is going to make our budget situation, our liquidity situation that much more challenging.”

Also at the UN, the head of the Palestinian delegation, Riyad Mansour, told UN reporters he had just learned about a decision by the secretary of state, Marco Rubio, to revoke the visas of Palestinian Authority and Palestine Liberation Organization officials ahead of the annual General Assembly meetings next month.

“We will see exactly what it means and how it applies to any of our delegation, and we will respond accordingly,” he said.

In a statement earlier Friday, the Department of State said it is “in our national security interests to hold the PLO and PA accountable for not complying with their commitments, and for undermining the prospects for peace.”

“Before the PLO and PA can be considered partners for peace, they must consistently repudiate terrorism — including the October 7 massacre — and end incitement to terrorism in education, as required by U.S. law and as promised by the PLO.”

The statement did not specify how many visas were being suspended.