US President Donald Trump cancelled a planned meeting with Russian President Vladimir Putin on the sidelines of a weekend G20 summit, citing the crisis in Ukraine.

Trump tweeted the decision on Thursday after he departed Washington for the summit in Argentina.

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"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 wrote on Twitter. "I look forward to a meaningful Summit again as soon as this situation is resolved!"

The sudden cancellation came just an hour after Trump told reporters he would likely sit down with Putin and it was "a very good time to have the meeting."

The Kremlin said it had not been notified of the cancellation and first learned about it from the tweet.

Dmitry Peskov, Putin's spokesman, said the cancellation meant that Putin would have "a couple more hours" for "useful meetings" with other leaders.

On Sunday, Russia fired on three Ukrainian naval ships off the coast of Crimea and seized the vessels and 24 Ukrainian seamen. 

Russia said Ukraine didn't have permission to pass through the Kerch Strait, which separates mainland Russia from annexed Crimea. Ukraine said its ships were in international waters. In response, Ukraine imposed martial law.

cw/msh (AFP, AP, dpa, Reuters)