US President Barack Obama said today Washington is prepared to work with Russia and Iran to try to end the more than four-year war in Syria that has spawned Islamic State militants.

"The United States is prepared to work with any nation, including Russia and Iran, to resolve the conflict," Obama told the annual gathering of world leaders at the United Nations. "But we must recognize that there cannot be, after so much bloodshed, so much carnage, a return to the pre-war status quo."

"There is no room for accommodating an apocalyptic cult like ISIL (Islamic State) and the United States makes no apology for using our military as part of a broad coalition to go after it," he said. He described Syrian President Bashar al-Assad as a tyrant.

Obama was among some 200 world leaders who have met at the United Nations for marathon talks to negotiate an ambitious roadmap aimed at tackling the world's most troubling problems.

It is the most sweeping agenda ever of global goals to combat poverty, inequality and climate change, capping years of debate described by the United Nations secretary-general as "a to-do list for people and planet."

But the summit also took place under the shadow of a migrant crisis in Europe caused in no small part by ongoing conflicts in the Middle East especially the war in Syria.