US president Donald Trump has suggested he may cancel his planned meeting with Vladimir Putin over Russia's seizure of three Ukrainian naval ships last weekend.
In an interview with the Washington Post, Mr Trump said he would be receiving a "full report" from his national security team on Russia's recent actions in eastern Ukraine and the Black Sea. He said he would decide on a course after that.
The two leaders are due to meet on the sidelines of the G20 summit in Buenos Aires later this week.
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“Maybe I won’t have the meeting,” he said. “Maybe I won’t even have the meeting.”
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He added: “I don’t like that aggression. I don’t want that aggression at all.”
The comments were Mr Trump’s strongest to date in condemnation of Russia’s recent actions in Ukraine, where tensions are flaring.
But White House aides were still planning for the Putin meeting after Mr Trump's comments.
The meeting between the two presidents is set to be just one of several high-profile foreign policy engagements for the US leader on a two-day visit to Argentina.
Mr Trump is also set to meet Chinese president Xi Jinping over dinner this weekend, in what may be a pivotal session in determining if and how the ongoing trade dispute between their two countries could be resolved.
On Tuesday, the White House warned Mr Xi against trying to wait out Mr Trump in the ongoing talks, suggesting the Chinese economy was not as resilient to a trade war as would be the US.
The warning from Larry Kudlow, director of the National Economic Council, came ahead of the two leaders' high-stakes sit-down on Saturday evening.
Over the last year, the two countries have levied a series of tariffs on hundreds of billions of dollars of imports from one another, with the latest round of US duties set to go into force in the new year.
National Security adviser John Bolton said Mr Trump will also be meeting German chancellor Angela Merkel, Japanese prime minister Shinzo Abe, Argentine president Mauricio Macri, South Korea's Moon Jae-in, Turkish president Recep Tayyip Erdogan and Indian prime minister Narendra Modi.
White House spokeswoman Sarah Huckabee Sanders called the trip an opportunity for the president to cement relations with other world leaders and advance a global economic system based on "free, fair and reciprocal trade".