WASHINGTON (Reuters) – U.S. President Donald Trump on Thursday suddenly canceled a meeting with Russian President Vladimir Putin scheduled for this week’s Group of 20 industrialized nations summit in Argentina, citing the current Ukraine crisis.

“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. I look forward to a meaningful Summit again as soon as this situation is resolved!” Trump tweeted after departing for the G20 summit.

Trump’s tweet was a sudden turnaround. Roughly an hour earlier, he had told reporters he would probably meet with Putin at the summit and said it was “a very good time to have the meeting.”

But Trump had also said he would get a final report during the flight to Argentina on the tension in the region after Russia seized Ukrainian vessels near Crimea on Sunday.

Differences over Ukraine, as well as Moscow’s role in the civil war in Syria, have been an irritant in U.S.-Russian relations for years.

The administration of former President Barack Obama imposed sanctions on Russia for its annexation of Crimea from Ukraine in 2014. That in part brought ties between Washington and Moscow to their lowest point since the end of the Cold War.

Since then, the United States has investigated Russia’s possible interference in the 2016 presidential election that Trump won. Russia has denied meddling and Trump has repeatedly said there was no collusion.

(Reporting by Makini Brice; Writing by Lisa Lambert; Editing by Alistair Bell; Editing by Chizu Nomiyama and Alistair Bell)