US President Donald Trump has abruptly cancelled a planned meeting with Russian President Vladimir Putin in Argentina, registering his disapproval of Russia's treatment of Ukraine and casting new uncertainty over US-Russian ties.
Mr 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.
Mr Trump's former lawyer Michael Cohen has pleaded guilty 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," Mr Trump tweeted.
"I look forward to a meaningful Summit again as soon as this situation is resolved!"
Mr Trump's tweet, from aboard Air Force One shortly after takeoff from Washington on the way to Buenos Aires for a G20 summit, was a sudden turnaround.
Roughly an hour earlier, he had told reporters he would probably meet with Mr Putin at the summit and said it was "a very good time to have the meeting."
Mr Trump's cancelation marked a dramatic change in tone. In the past he has repeatedly stressed his desire for a warm relationship with Mr Putin.
When the two leaders met in Helsinki in July, Mr Trump refused to criticize Mr 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," Mr Peskov said.
Mr 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.
Mr Bolton has pushed for Mr Trump to withdraw the United States from the Cold War-era Intermediate-Range Nuclear Forces treaty over what US officials called Moscow's violation of it.
US officials are watching with alarm the crisis surrounding Ukraine.
Ukrainian President Petro Poroshenko has accused Putin of wanting to annex his entire country and called for NATO to deploy warships to a sea shared by the two nations.
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.
Ukraine 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 it of important transit revenue.