President Trump set the lowest possible bar for his meeting with President Vladimir V. Putin of Russia on Friday, declaring that “probably in the first two minutes I’ll know exactly whether or not a deal can get done,” and insisting he was ready to walk away from the talks and let the two sides continue to fight it out.

In a rambling news conference, Mr. Trump reiterated that he planned to negotiate what he called “land swaps” and batted away the statements over the weekend by Ukraine’s president, Volodymyr Zelensky, that his country’s Constitution prohibited him from giving away land to an invader.

In describing the meeting, Mr. Trump told reporters that “I’m going to Russia on Friday,” and repeated a version of the same statement several minutes later. In fact, the meeting is set to take place in Alaska, which has not been part of Russia since 1867, when it was sold to the United States for $7.2 million.

“I’m going to see what he has in mind,” Mr. Trump said of Mr. Putin, whom he has talked to over secure lines at least five times since he took office in January. He said he would judge “if it’s a fair deal.”

He added, “I may leave and say good luck, and that’ll be the end.”

But Mr. Trump’s own description of his goals for the negotiation on Friday, the most high-stakes international meeting yet in his second term, were telling — as much for what he omitted as for what he included. And that is what worries both the Ukrainians and Washington’s European allies, who have committed to keep arming Ukraine no matter the outcome in Anchorage.

Mr. Zelensky, who so far has not been invited to join the talks in Alaska, has said that any agreement must start with a some kind of truce or cease-fire so that negotiations were not being conducted amid continued air attacks and territorial grabs. Mr. Trump has not stipulated that a truce must come first.