US President Donald Trump has said he may cancel a long-awaited summit with his Russian counterpart Vladimir Putin after a confrontation at sea between Russia and Ukraine led Kiev to warn of the threat of “full-scale war”.

Trump is scheduled to meet Putin at the G20 summit in Buenos Aires at the end of this week, but warned it would depend on the results of a report being prepared by his national security advisers about Russia’s seizure of three Ukrainian ships.

“Maybe I won’t have the meeting. Maybe I won’t even have the meeting,” he told the Washington Post on Tuesday.

Ukrainian President Petro Poroshenko earlier warned that the standoff could herald more drastic developments as tensions escalate between the ex-Soviet neighbours.

“I don’t want anyone to think this is fun and games. Ukraine is under threat of full-scale war with Russia,” Poroshenko said.

The number of Russian units deployed along the Ukraine-Russian border has “grown dramatically” while the number of Russian tanks has tripled, Poroshenko said, citing intelligence reports but giving no precise timescale for the buildup.

He spoke after Russian forces seized three of Kiev’s ships off the coast of Crimea on Sunday and captured 24 Ukrainian sailors.

On Tuesday, a court in Simferopol, the main city in Russian-annexed Crimea, ordered 12 of the sailors to be held in pre-trial detention for two months. Three hospitalised sailors were also formally detained for two months. The rest are to appear in court on Wednesday. The move stoked already high tensions between Moscow and Kiev, as Russia continues to accuse the sailors of crossing illegally into Russian waters and of ignoring warnings from its border guards.

Kiev has demanded the release of the sailors and urged Western allies to impose further sanctions on Moscow.

The US State Department has termed Russia’s actions “a dangerous escalation” and said Washington wants to see “our European allies doing more to assist Ukraine.”