Trump has a packed schedule of meetings with world leaders at the summit

It currently includes ones with Russian President Vladimir Putin

U.S. president says he could call it off once he has a full report on Russia's seizure of three Ukrainian ships

President Donald Trump is threatening to call off a meeting with Vladimir Putin at the G20 over Russian 'aggression' in the Ukraine in a major break with the leader he has tried to befriend.

Trump said he's awaiting a 'full report' on Russia's seizure of three Ukrainian ships. The national security briefing will be 'determinitive' on whether he moves ahead with a planned meeting in Argentina with Putin.

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'Maybe I won’t have the meeting. Maybe I won’t even have the meeting,' Trump told the Washington Post in an interview. 'I don’t like that aggression. I don’t want that aggression at all.'

President Donald Trump is threatening to call off a meeting with Vladimir Putin at the G20 over Russian 'aggression' in the Ukraine in a major break with the leader he has tried to befriend

U.S. ambassador Nikki Haley furiously condemned Russia at the United Nations this week for taking the ships their crews hostage.

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'In the name of international peace and security, Russia must immediately cease its unlawful conduct and respect the navigational rights and freedoms of all states,' she said on Monday morning.

Haley did not threaten new sanctions on Russia, and neither did Trump.

She merely stated: 'As President Trump has said many times, the United States would welcome a normal relationship with Russia. But outlaw actions like this one continue to make that impossible.'

Trump said he's awaiting a 'full report' on Russia's seizure of three Ukrainian ships. The national security briefing will be 'determinitive' on whether he moves ahead with a planned meeting in Argentina with Putin

U.S. Ambassador to the United Nations Nikki Haley blasted Russia during an emergency Security Council session, saying Moscow took an 'outlaw action' on Sunday by firing on and seizing three Ukrainian ships

Sanctions on Russia for the 2014 incursion into Ukraine are still in place.

'The United States will maintain its Crimea-related sanctions against Russia. Further Russian escalation of this kind will only make matters worse,' Haley said on Monday. 'It will further undermine Russia's standing in the world. It will further sour Russia´s relations with the U.S. and many other countries,' Haley said.

Six seamen were wounded when the Russian coast guard opened fire on and then seized three Ukrainian ships during the unprecedented stand-off.

Footage has emerged of the Russian coast guard vessel ramming a Ukrainian navy tugboat under the orders 'squash him.' Crew on the Russian ship were told to 'hold on tight' as it plowed into the Ukrainian tug.

The clash has raised fears of a wider military escalation and both NATO and the UN Security Council were holding emergency meetings to discuss the crisis on Monday.

The 15-member UN Security Council has met dozens of times about the Russia-Ukraine crisis, but its powers have been rendered toothless as the nation is one of the panel's five permanent members – status that comes with the power to veto resolutions.

Now, Ukraine is preparing to defend itself from a possible invasion, even as Russia accuses Kiev of plotting with its Western allies to provoke a conflict.

The Ukraine has imposed martial law on its borders in response.

Britain and France immediately castigated Moscow, with British Prime Minister Theresa May called it an 'act of aggression' that provides 'further evidence of Russia's destabilising behaviour in the region.'

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Spain and Germany also joined EU calls for Russia to release the Ukrainian sailors and ships seized in the Sunday standoff.

Trump said Tuesday that he doesn't like the 'aggression' and might not have a planned meeting with Putin.

He has a full slate of meetings with foreign leaders when he's at the G20 in Argentina this week. None, save for a dinner with China's Xi Jinping, will be as closely watched, however, than his sit-down with the Russian leader, especially in the wake of the violence.

The two leaders will talk about security issues, arm controls issues, and regional issues including Middle East, the president's National Security Advisor John Bolton said Tuesday before Trump's interview with the Post was released.

Trump and Putin spoke briefly in France when both men were present for a celebration of the 100th anniversary of the armistice that ended World War I. But they have not had a formal sit down since their July summit in Helsinki, Finland.

There, Trump backed Putin's assertion that Moscow did not meddle in the 2016 presidential election, contradicting conclusions from every U.S. intelligence agency.

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The president later walked back his remarks, saying he meant to say he had no reason to believe U.S. intelligence was wrong and that he had no reason to believe that Putin was lying, as he played clean-up on a poorly-panned news conference.