There were frosty relations between Barack Obama and Vladimir Putin as they met at a UN Summit to discuss Syria.
The US President said he thought Syrian President Bashar al-Assad was a "tyrant" who was behind the four-year civil war in which at least 200,000 people have died.
While Russian premier Putin told the gathering of world leaders there was no alternative to cooperating with Assad's military to defeat the so-called Islamic State.
The pair shook hands and clinked glasses at a lunch hosted by UN Secretary-General Ban Ki-moon.
Fifty countries have pledged over 30,000 new troops and police for possible deployment on United Nations peacekeeping missions, US president Barack Obama has said.
More than 50 countries, from Bangladesh to Colombia, from Finland to China, are making commitments totaling more than 30,000 new troops and police.
Mr Obama was speaking at a summit of world leaders organised by the US to boost global support for UN peacekeeping missions.
The Syrian crisis has dominated the agenda throughout the meetings in New York.
Barack Obama has said he is prepared to work with Russia and Iran and any other world leaders to bring an end to the Syria crisis.
Addressing the UN General Assembly, the US President said there can be no return to the status quo in Syria under "tyrant" President Bashar al-Assad.
However Russian president Vladamir Putin later told the Assembly it was an "enormous mistake" to refuse to work with Assad's government and its armed forces.
ITV News Washington Correspondent Robert Moore reports:
Russian president Vladamir Putin has said it is an "enormous mistake" to refuse to work with the Syrian government.
Addressing the UN General Assembly, Putin said: "We think it is an enormous mistake to refuse to cooperate with the Syrian government and its armed forces who are valiantly fighting terrorism face to face.
"We should finally acknowledge that no one but President Assad's armed forces and (Kurdish) militia are truly fighting the Islamic State and other terrorist organizations in Syria."
He called for the creation of a broader international anti-terrorist coalition including Muslim states as key members.
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