Straining for a solution to Syria's civil war, President Barack Obama on Monday said the United States is willing to work with Russia, as well as Iran to achieve a "managed transition" to remove Syrian leader Bashar Assad from power.

Syria cannot "return to the pre-war status quo," Obama declared during his annual address to the United Nations General Assembly.

The president's remarks underscore the tensions between the US and Russia, Assad's strongest ally. Assad's future was expected to be a top issue during a rare face-to-face meeting late Monday between Obama and Russian President Vladimir Putin.

Putin was to address the UN General Assembly after Obama. He was expected to argue that Assad's military is the most capable force for fighting the Islamic State '€” the extremist group with key strongholds in Syria and Iraq '€” and therefore needs to be strengthened.

Obama rejected Putin's continued support for Assad, saying that simply arguing that the "alternative is surely worse" is not a solution.

Addressing another area of tension with Russia, the US president defended Western sanctions against Moscow for its actions supporting rebels in Ukraine.

"If that happens without consequences in Ukraine, it could happen to any nation gathered here today," Obama said.

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