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... "I have ordered two Nuclear Submarines to be positioned in the appropriate regions, just in case these foolish and inflammatory statements are more than just that," Trump said in a social media post that called Medvedev's statements highly provocative.

He said he ordered the submarines moved "just in case these foolish and inflammatory statements are more than just that. Words are very important, and can often lead to unintended consequences, I hope this will not be one of those instances."

Trump and Medvedev, who is deputy chairman of Russia's Security Council, traded taunts in recent days after Trump on Tuesday said Russia had "10 days from today" to agree to a ceasefire in Ukraine or be hit, along with its oil buyers, with tariffs.

Moscow, which has set out its own terms for peace in Ukraine, has shown no sign that it will comply with Trump's deadline. ...