UNITED NATIONS -- World leaders witnessed simmering tensions between President Barack Obama and Russian counterpart Vladimir Putin Monday as the two jousted in UN speeches over the conflict in Ukraine and how to resolve the crisis in war-torn Syria.

The public sparring occurred hours before they met to discuss both issues that have come to symbolize their ideological split.

Obama, in a 43-minute address, praised the UN and global rule of law but also chided China, Iran and Syria for ignoring diplomacy in recent spats with other nations, instead choosing an outdated "might makes right" approach.

"We, the nations of the world, cannot return to the old ways of conflict and coercion," Obama said. "We cannot look backward. We live in an integrated world, one in which we all have a stake in each other's success."

He said Russia's role in Ukraine and its backing of the president of Syria is not unlike the aggressive actions of China in the South China Sea over territory and Iran's support for anti-Western elements in the Middle East -- they all have violated the international order.

"Some major powers assert themselves in ways that contravene international law," Obama told the world leaders. "We're told that such retrenchment is required to beat back disorder -- that it's the only way to stamp out terrorism. In accordance with this logic, we should support tyrants like Bashar al-Assad."

Putin followed Obama and urged the world to stick by Assad as the only viable option for defeating the Islamic State.

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Putin also heaped praise on the UN's history of upholding international law and resolving crises ranging from border disputes to famine and, in a snub to Obama and his other Western detractors, held up Assad's administration as the "legitimate government" of Syria.

"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," he said. "We should finally acknowledge that no one but President Assad's armed forces and Kurd militia are truly fighting the Islamic State and other terrorist organizations in Syria."

Obama and Putin's disparate views of the grim situation in Syria left little indication of how the two countries might work together to end a conflict that has killed more than 250,000 people and resulted in millions of refugees.

The two leaders held a private meeting Monday night where they discussed Ukraine and Syria, according to a U.S. official. The official, who asked to remain anonymous, noted that Putin, when prompted, said he would not rule out airstrikes to combat ISIS.

Obama's address was his second in as many days at the UN.

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Sunday, he said the United States supports the UN's 17 sustainable development goals, which emphasize reducing poverty and disease while conserving the environment between now and 2030 and later yesterday, he convened the Leaders' Summit on Peacekeeping where more than 50 heads of state and government pledged up to 30,000 troops, police and civilian personnel to the 125,000 UN peacekeepers stationed at global hot spots.

"We are here today, together, to strengthen and reform UN peacekeeping because our common security demands it," Obama said.

The meeting fell in line with Obama's earlier address that put a premium on a multilateral, diplomatic approach to solving world conflicts -- with room for Russia's involvement.

He cited the Iran nuclear deal as an example of success through cooperation among the five members of the Security Council -- Russia, the United States, France, Great Britain and China -- and Germany and the European Union.

"And if this deal is fully implemented, the prohibition on nuclear weapons is strengthened, a potential war is averted," he said. "Our world is safer. That is the strength of the international system when it works the way it should."

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With The Associated Press