ABOARD AIR FORCE ONE (Reuters) - U.S. President Donald Trump on Thursday abruptly canceled a planned meeting with Russian President Vladimir Putin in Argentina, registering his disapproval of Russia’s treatment of Ukraine and casting new uncertainty over U.S.-Russian ties.

Trump said he pulled out due to tensions over Russian forces opening fire on Ukrainian navy boats and then seizing them and their crew on Sunday near Crimea, which Russia annexed from Ukraine in 2014.

His decision also comes as a federal investigation into his 2016 election campaign’s ties to Russia is intensifying. Trump’s former lawyer Michael Cohen pleaded guilty on Thursday to lying to Congress about a proposed Trump real estate project in Russia.

“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.

Trump’s tweet, from aboard Air Force One shortly after takeoff from Washington on the way to Buenos Aires for a Group of 20 summit, 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.”

Trump’s cancellation marked a dramatic change in tone. In the past he has repeatedly stressed his desire for a warm relationship with Putin. When the two leaders met in Helsinki in July, Trump refused to criticize Putin and drew fire at home.

His announcement appeared to catch Moscow by surprise, with Russian officials getting their first news of it from the media.

Kremlin spokesman Dmitry Peskov told RIA news agency that the Kremlin regretted Trump’s decision and said Russia is ready for contact with him.

“A cancellation means that the discussion on key international issues is being postponed indefinitely,” Peskov said.

Trump based his move on advice from senior advisers who have taken a harsher tone toward Russia than he has. He was briefed on Air Force One by Secretary of State Mike Pompeo, White House chief of staff John Kelly and national security adviser John Bolton, White House spokeswoman Sarah Sanders said.

Bolton has pushed for Trump to withdraw the United States from the Cold War-era Intermediate-Range Nuclear Forces treaty over what U.S. officials called Moscow’s violation of it.

U.S. officials are watching with alarm the crisis surrounding Ukraine.

Ukrainian President Petro Poroshenko on Thursday accused Putin of wanting to annex his entire country and called for NATO to deploy warships to a sea shared by the two nations.

Holding talks with Putin now could represent bad optics for the White House at a time when the president is under scrutiny over prior plans to build a Trump Tower in Moscow.

Cohen pleaded guilty on Thursday to lying to Congress about the proposed Trump Organization skyscraper in Moscow, prompting Trump to lash out at Cohen as a “liar” and “weak person.”

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.

U.S. President Donald Trump talks to reporters as he departs on travel to the G20 Summit in Argentina from the White House in Washington, U.S., November 29, 2018. REUTERS/Jonathan Ernst

Democratic U.S. Senator Bob Menendez of New Jersey said Trump was missing an “opportunity to redeem himself, stand up for American values, stand up for international law, stand up for our own national security interests, and he had that opportunity and instead he’s abdicating it.”

“It is ironic that this President cannot find his spine to confront Vladimir Putin but can challenge the closest allies the United States has across the globe,” Menendez said.

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 election that Trump won. Russia has denied meddling and Trump has repeatedly said there was no collusion.

Russia seized three Ukrainian navy ships and their crews on Sunday near Crimea over what it said was their illegal entry into Russian waters, which Ukraine denies.

Some of Ukraine’s Western allies have raised the possibility of imposing new sanctions on Russia over the episode, which could deliver a blow to the Russian economy.

Kiev is aiming to gain Western support for more economic sanctions against Moscow, secure tangible Western military help, and rally opposition to a Russian gas pipeline that threatens to deprive Ukraine of important transit revenue.