BRISBANE - United States President Barack Obama said that Russia would remain isolated by the international community if President Vladimir Putin continued to violate international law in Ukraine, in some of his toughest remarks yet on the crisis.
Mr Putin came under intense pressure from other leaders at the weekend's Group of 20 summit over his government's backing of pro-Russian rebels in Ukraine, where a conflict has killed more than 4,000 people this year.
Mr Obama, who described his meetings with Mr Putin last week as "blunt and business-like", said that he had made it clear that 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," the US leader said at a press conference yesterday.
On Saturday, Western leaders warned Mr Putin that he risked more sanctions if he failed to end his country's backing of the separatists. Russia has denied any involvement.
Yesterday, Mr Obama said more sanctions were a possibility.
"At this point, the sanctions we have in place are biting plenty good," he said after the G-20 meeting in Brisbane. "We retain the capability and we have our teams constantly looking at mechanisms in which to turn up additional pressure as necessary."
Mr Putin, who left the summit early because he needed to "sleep at least four to five hours", praised the "constructive atmosphere" in Brisbane and said there was a "good chance of a resolution" to the Ukraine crisis.
On Syria, Mr Obama warned President Bashar Al-Assad not to allow his military to engage US warplanes conducting operations against Islamic State of Iraq and Syria (ISIS) militants on its territory.
"We have communicated to the Syrian regime that when we operate going after ISIL in their airspace, they would be well advised not to take us on," Mr Obama said, using another name for ISIS.
He ruled out a political solution to the civil war in Syria that would leave Mr Assad in power and denied reports that his administration was conducting a formal review of its military policy in the country.
He accepted, however, that it was the "nature of diplomacy" that Washington would eventually have to deal with some of its rivals to bring peace to the war-torn country.
"At some point, the people of Syria and the various players involved, as well as the regional players, Turkey, Iran, Assad's patrons like Russia, are going to have to engage in a political conversation," he said.