Prime Minister Viktor Orban is jeopardizing Hungary’s position as a trusted NATO ally, the U.S. ambassador to Budapest warned on Thursday, with “its close and expanding relationship with Russia,” and with “dangerously unhinged anti-American messaging” in state-controlled media.

The ambassador, David Pressman, has for months criticized Mr. Orban for effectively siding with President Vladimir V. Putin of Russa over the war in Ukraine, but his latest remarks sharply ratcheted up tensions and indicated that trust in Hungary among NATO allies had collapsed.

Hungary is “an ally that behaves unlike any other” and is “alone on the defining issue of European security of the last quarter century, Russia’s war in Ukraine,” Mr. Pressman said in a speech in Budapest marking the 25th anniversary of Hungary’s admission to the Western military alliance.

“We will have to decide how best to protect our security interests, which, as allies, should be our collective security interests,” he added.

The speech followed a visit last week by Mr. Orban, a darling of MAGA Republicans in the United States, to Donald J. Trump at the former president’s home and members-only club in Florida. After their meeting, Mr. Orban claimed in an interview with Hungarian state television that Mr. Trump had outlined to him a “pretty detailed plan” for ending the war in Ukraine that would involve an abrupt halt to United States aid to Russia’s embattled neighbor.

Such a plan closely parallels what Mr. Orban has been advocating for the European Union — a suspension of all financial and military support for Ukraine, and a policy of pushing the government in Kyiv into immediate peace negotiations with Moscow.