President Trump said on Wednesday that he had a “lengthy and highly productive phone call” with President Vladimir V. Putin of Russia, characterizing it as the beginning of a negotiation to end the war in Ukraine.

It was the first confirmed conversation between the two men during Mr. Trump’s second term, coming as Mr. Trump has made clear to advisers that finding a U.S.-backed end to war that Russia began is a priority for his administration.

“We discussed Ukraine, the Middle East, Energy, Artificial Intelligence, the power of the Dollar, and various other subjects,” Mr. Trump wrote in a social media post.

“We each talked about the strengths of our respective Nations, and the great benefit that we will someday have in working together,” Mr. Trump added. “But first, as we both agreed, we want to stop the millions of deaths taking place in the War with Russia/Ukraine.” (An estimated several hundred thousand deaths have occurred in the conflict, not millions.)

The U.S. president said he planned to inform President Volodymyr Zelensky of Ukraine that both countries planned to “have our respective teams start negotiations immediately.” Mr. Zelensky’s office later said that the Ukrainian president spoke with Mr. Trump for an hour.

However, Mr. Trump did not say in his social media post how Mr. Zelensky would factor into the negotiations that he and Mr. Putin were setting in motion. Mr. Trump has long been skeptical of Ukraine and has never warmed to Mr. Zelensky.