President Trump abruptly called off plans to meet with Russian President Vladimir Putin because of escalating military tension with Ukraine, the first in a rush of last-minute schedule changes made Thursday aboard Air Force One.

Mr. Trump tweeted out the cancellation while flying to Argentina for the Group of 20 summit, contradicting his comments just 40 minutes earlier that he probably would go ahead with the meeting.

More rearranging of the president’s schedule for the two-day summit would soon follow.

Citing a skirmish Sunday in which Russia’s coast guard fired on and seized three vessels from Ukraine, a U.S. ally, Mr. Trump said it would be “best for all parties” if he snubbed the Russian leader.

“Based on the fact that the ships and sailors have not been returned to Ukraine from Russia, I have decided it would be best for all parties concerned to cancel my previously scheduled meeting in Argentina with President Vladimir Putin,” he tweeted. “I look forward to a meaningful Summit again as soon as this situation is resolved!”

During the flight, White House press secretary Sarah Huckabee Sanders said Mr. Trump’s formal meetings with Turkey and South Korea also had been canceled. Mr. Trump instead will speak informally with the leaders at the summit, she said.

The president’s summit schedule still includes formal meetings with leaders of Argentina, Germany, Japan and India.

The biggest meeting, however, will be a Saturday night dinner with Chinese President Xi Jinping where trade will top the agenda.

Mrs. Sanders did not say why the meetings with South Korea and Turkey were downgraded. Regardless, Mr. Trump’s shifting schedule provided an inauspicious start on his trip abroad.

Mr. Trump’s tweet took the Kremlin by surprise.

Putin spokesman Dmitry Peskov told Russian news services that the Kremlin learned of the cancellation from Mr. Trump’s tweet and had not received formal notification.

Mr. Peskov said that if the meeting was off, then Mr. Putin would have more time for “useful meetings” with other leaders.

Like Mr. Trump, Mr. Putin was on his way to Argentina.

As he left the White House, Mr. Trump said he wanted to read the final report about the incident aboard his flight.

Mr. Trump made the decision to cancel the meeting aboard Air Force One after he conferred in person with Secretary of State Mike Pompeo and Chief of Staff John F. Kelly and by phone with National Security Adviser John R. Bolton, who is in Brazil.

Mrs. Sanders described the decision-making process to reporters traveling with the president.

The process wasn’t pretty, but the outcome was the right call, said Georgetown University national security professor Christopher Swift.

“It’s a waste of everyone’s time for these two guys to get together in the same room,” he said.

He noted that neither side likely would have budged on the big issues: Washington’s desire for eased military tension between Russia and Ukraine and Moscow’s push for sanctions relief.

The only reason for Mr. Trump to go to the meeting would be to read Mr. Putin the riot act and threaten enhanced U.S. military presence in the region, he said, which is a step the administration does not appear ready to take.

“You can’t let the Russians set the agenda, and sometimes talking for the sake of talking isn’t a good outcome,” Mr. Swift said.

The incident near Crimea in the Kerch Strait, which connects the Black Sea to the Sea of Azov, sparked fears of war as Kiev and Moscow blamed each other.

Ukrainian leaders warned Tuesday about the risk of a full-scale military confrontation with Russia.

Sen. Robert Menendez of New Jersey, the top Democrat on the Senate Foreign Relations Committee, said Mr. Trump made the wrong move by canceling the meeting.

“This was [the president’s] opportunity to redeem himself, stand up for American values, stand up for international law, stand up for our own national security interests, and he had that opportunity and instead he’s abdicating it,” he said on CNN. “It is ironic that this president cannot find his spine to confront Vladimir Putin but can challenge the closest allies the United States has across the globe.”

Mr. Trump weathered similar criticism for being soft on Mr. Putin when the two leaders met at a July summit in Helsinki.

Relations between Washington and Moscow have been badly strained since Russia’s annexation of Crimea from Ukraine in 2014.

The bad blood between the U.S. and Russia only worsened with Kremlin meddling in the 2016 presidential election and its implication in the nerve agent poisoning of a former Russian spy in the United Kingdom.

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