Barack Obama and Vladimir Putin have been sparring in speeches to the United Nations, hours before the US and Russian presidents have a rare one-one-one meeting
In his remarks to world leaders, Mr Obama rejected Mr Putin's support for Syrian President Bashar al Assad, saying that simply arguing the "alternative is surely worse" is not a solution.
He told the General Assembly that Mr Assad's slaughter of his own people was "an assault on all our humanity".
The US President did make an overture to Mr Putin by offering to work with Russia, and Iran, for a "managed transition" to remove Mr Assad.
But he emphasised there could be no "return to the pre-war status quo" after the conflict, which has raged for more than four years and claimed at least 200,000 lives.
However, Mr Putin said in his speech that it was an "enormous mistake" not to back Mr Assad, whom he depicted as a bulwark against the Islamic State.
The Russian leader said the jihadist group's ranks were being filled with members of Iraq's US-disbanded military and so-called moderates backed by Western powers.
Mr Putin took clear aim at the US, lambasting policies "based on self-conceit and belief in one's own exceptionality and impunity".
He said the international community was not learning from its mistakes, citing the example of Western backing for revolutions in the Middle East.
Mr Obama told world leaders there must be consequences for Russia's annexation of Crimea.
"We cannot stand by while the sovereignty and territorial integrity of a nation is flagrantly violated," he said.
"If that happens without consequences in Ukraine, it could happen to any nation gathered here today."
The US and Russia have been unable to agree even on the agenda of their Monday afternoon meeting on the sidelines of the annual summit in New York City.
The White House said it would focus on Ukraine, but the Kremlin said they would discuss Syria and the fight against the Islamic State.
The two leaders have rarely encountered one another since Mr Obama vowed to isolate Mr Putin in retaliation for Russia's actions in Ukraine.
In a US TV interview, Mr Putin said he had no plans "right now" to deploy combat troops in Syria, amid US concerns about a Russian military build-up.
