President Trump said on Monday that he was “disappointed” with President Vladimir V. Putin over the war in Ukraine, and said he would give Russia 10 to 12 days to end the conflict or face a new round of sanctions.

Mr. Trump had warned Moscow earlier this month that the United States would impose “very severe tariffs” if Russia did not agree to a peace deal within 50 days, but on Monday he said he would bring that deadline forward.

“I’m going to make a new deadline of about 10 or 12 days from today. There’s no reason in waiting,” Mr. Trump told reporters in Scotland during a meeting with Prime Minister Keir Starmer of Britain. “We just don’t see any progress being made.”

If the Kremlin does not comply, “it would be sanctions and maybe tariffs, secondary tariffs,” he said. Mr. Trump has repeatedly threatened to punish Russia over its escalating attacks in Ukraine but so far has not followed through.

The imposition of new tariffs on Russia would be a shift for the White House, which did not name the country when it imposed levies on a host of countries in April. And the threat to impose secondary tariffs — levies on any country that trades with Russia — could threaten trading partners, notably China and India, that import Russian oil and gas.

Mr. Trump returned to the White House in January, saying he could bring peace to Ukraine immediately, but the war has continued with Russian forces on the offensive in eastern and northern Ukraine. Initially, Mr. Trump appeared to side with Mr. Putin on the war, repeatedly criticizing President Volodymyr Zelensky of Ukraine and suspending supplies of weapons to Kyiv.