“We are working closely with our Mexican counterparts to confront rising border apprehension numbers — specifically, a 38 percent increase in families this month alone — directly and to ensure that those with legitimate claims have access to appropriate protections,” Ms. Waldman said.

A spokesman for the Mexican Embassy did not immediately respond Wednesday to a request for comment.

The plan, which has been debated internally for months, is part of a broader push by the Trump administration to redirect billions in foreign assistance to other priorities. The administration has yet to spend nearly $3 billion in foreign aid, money allocated last year by Congress with broad bipartisan support. Hundreds of millions of dollars meant to help stabilize Syria and support Palestinian schools and hospitals has already been redirected.

While the administration has made several announcements about not spending on priorities Congress intended, it has mostly kept quiet about what it will do with the money. But it has long been frustrated that Congress provides billions for foreign aid while refusing to fund its immigration priorities. The money will be transferred from the State Department to the Department of Homeland Security, and then sent to Mexico.

“Congress intended for this money to lift up communities dealing with crime, corruption and so many other challenges, not to expand this administration’s deportation crusade,” said Representative Eliot L. Engel of New York, the top Democrat on the House Foreign Affairs Committee. “I want answers about why the State Department thinks it can ignore Congress and dump more cash into deportation efforts. Until then, I’ll do whatever I can to stop this.”

The maneuver is the latest by the administration to reduce the number of immigrants crossing the southwestern border. The most prominent piece of the effort has been the “zero tolerance” policy to criminally prosecute any immigrant who enters the country without authorization. That led to the widely criticized practice of separating children from their parents at the border, which spurred a humanitarian and political crisis for Mr. Trump.