BRISBANE, Australia -- Russia's President Vladimir Putin faced a chorus of criticism Saturday at a world leaders' summit, where the escalating conflict between Russia and Ukraine threatened to overshadow talk of global economics.

European Union President Herman von Rompuy said that while the Ukraine conflict was not on the agenda of the Group of 20 wealthy and developing economies, the topic would be discussed between President Barack Obama and EU leaders in Brisbane ahead of a European Union decision on further sanctions against Moscow.

"We will continue to use all diplomatic tools including sanctions at our disposal," von Rompuy told reporters. "The EU foreign ministers will on Monday assess the situation on the ground and discuss possible further steps."

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"We need to avoid a return to a full-scale conflict," he said.

Putin received a less-than-warm welcome from Canadian Prime Minister Stephen Harper when he approached Harper for a handshake.

"I guess I'll shake your hand, but I have only one thing to say to you: You need to get out of Ukraine," Harper told Putin, according to the prime minister's spokesman, Jason MacDonald.

As for Putin's reaction? "I can say that he did not respond positively," MacDonald said in an email, declining to give further details.

In a speech to a Brisbane university audience, Obama said that the United States was a leading voice in opposing Russian aggression in Ukraine, which he described as "a threat to the world."

He referred to the shooting down on July 17 of a Malaysia Airlines jet by a missile suspected to have been fired by Russian-backed rebels in eastern Ukraine with the loss of 298 lives, including 38 Australian citizens and residents.

"As your ally and friend, America shares the grief of these Australian families and we share the determination of your nation for justice and accountability," Obama said.

Von Rompuy called for both sides to abide by the cease-fire agreement between Ukraine and the rebels that was signed in Minsk, Belarus, in September.

Russia must use its influence on the rebels to ensure they comply with the Minsk agreement, stop the flow of weapons and troops from Russia and withdraw Russian troops already in Ukraine, von Rompuy said.

U.S. Secretary of State John Kerry issued a similar call. The State Department said Kerry expressed "grave concern" about increased Russian support for the separatists and called for the implementation of the Minsk agreement, including a cease-fire, border monitoring, release of all hostages and a return to dialogue during a discussion with Russian Foreign Minister Sergey Lavrov.

Von Rompuy declined to comment on the likelihood of the EU deciding to ratchet up sanctions.

"Russia has still the opportunity to fulfill its Minsk agreements and chose the path of de-escalation, which could allow sanctions to be rolled back," he said. "If it does not do so however, we are ready to consider additional action."

Russia's military has also been flexing its muscles around the world this week.

Russia is stationing warships in waters off Australia's northeastern coast, prompting the Australian prime minister to angrily accuse Russia of trying to reclaim the "lost glories" of the Soviet Union.

On Wednesday, Russia announced it would conduct regular patrol missions from the Arctic Ocean to the Caribbean and the Gulf of Mexico. CBS correspondent David Martin reported that, even during Cold War times, the Soviet Union didn't fly bombers over the Gulf of Mexico, only surveillance and anti-submarine aircraft.

If Moscow were to make good on the threat now, a U.S. military official told Martin it would be a "very significant" development.