BRISBANE, Nov. 15 - The leaders of the United States, Japan and Australia lined up together against Russia on Sunday, vowing to oppose Russian incursions into Crimea during a rare trilateral meeting held at the G20 summit in Brisbane, Reuters reported.

President Barack Obama, Japanese Prime Minister Shinzo Abe and Australian Prime Minister Tony Abbott said they would oppose "Russia's purported annexation of Crimea and its actions to destabilize eastern Ukraine," and were committed to "bringing to justice those responsible for the downing of Flight MH17."

Obama also said on Sunday that Russia would remain isolated by the international community if President Vladimir Putin continued to violate international law in Ukraine, some of his toughest remarks yet on the crisis.

Putin has come under intense pressure from other G20 leaders over his government's backing for pro-Russian rebels in Ukraine, where a conflict has killed more than 4,000 people this year.

Obama, who described his meetings with Putin this week as "blunt and business-like", said that he had made it clear Russia would remain isolated if it continued to provide support to separatist rebels.

"We are also very firm on the need to uphold core international principles. And one of those principles is that you don't invade other countries or finance proxies and support them in ways that break up a country that has mechanisms for democratic elections," Obama said at a press conference.

On Saturday, Western leaders warned Putin that he risked more sanctions if he failed to end his country's backing for the separatists. Russia has denied any involvement.

Obama said more sanctions were a possibility.

"At this point the sanctions we have in place are biting plenty good," said Obama, speaking after the Group of 20 meeting in Brisbane, Australia. "We retain the capability, and we have our teams constantly looking at mechanisms in which to turn up additional pressure as necessary." (rt/ez)