He once expressed hope that President Vladimir V. Putin of Russia would be “my new best friend,” but with friends like him, President Trump hardly needs enemies.

Russia’s armed seizure of three Ukrainian ships in the days before the two leaders are scheduled to meet on the sidelines of the Group of 20 meeting in Buenos Aires has threatened to upend Mr. Trump’s plans to forge a fruitful relationship with Mr. Putin and may deepen the renewed tension between the former Cold War rivals.

After two years of going out of his way to avoid public criticism of his Russian counterpart, Mr. Trump has inched closer to a breaking point, declaring that he was “not happy” about the confrontation in the Sea of Azov and suggesting that he might even call off his session with Mr. Putin scheduled for Saturday.

As of Wednesday, the Kremlin said it still expected the meeting to proceed and the White House did not indicate otherwise. But if the two leaders do get together, Mr. Trump will face pressure to more forcefully condemn Russian aggression or risk once again looking deferential to Mr. Putin as he did in Helsinki this summer when he seemed to accept the Russian’s word that he had not interfered in the 2016 presidential election.