Just 24 hours after President Trump threatened Russia with financial penalties over the war in Ukraine, he seemed unsure on Tuesday about whether the strategy would even work.

Speaking to reporters aboard Air Force One, Mr. Trump said that in 10 days, the United States may have to impose “tariffs and stuff.”

“I don’t know if it’s going to affect Russia, because he wants to, obviously, probably keep the war going,” Mr. Trump said, referring to President Vladimir V. Putin. “But we’re going to put on tariffs and the various things you put on. It may or may not affect them. But it could.”

Mr. Trump once suggested he could end the war in Ukraine by appealing to Mr. Putin, man to man. He tried to rewrite the history of Russia’s invasion by casting Ukraine and its president, Volodymyr Zelensky, not as victims but as villains. But now, as Mr. Trump’s frustration over the conflict grows, his threats have raised questions about how much leverage the United States has with Moscow — and whether Mr. Trump is willing to use it.

“Trump seems to be realizing what a lot of us observed from early on — that Zelensky is not the problem,” said Matt Duss, the executive vice president at the Center for International Policy. “Ukraine is not the problem. Putin is the problem.”

Mr. Trump “obviously had great confidence in his own deal-making abilities,” Mr. Duss added, “but reality seems to be hitting.”