Islamic State is an "apocalyptic cult" whose mass slaughters are an "assault on all humanity", US President Barack Obama told the UN General Assembly on Monday.

Speaking before delegates from all UN nations and UN Secretary-General Ban Ki-moon in New York, Obama said there are no easy answers to the crisis, but "there is no room for accommodating an apocalyptic cult like ISIL, and the United States makes no apologies for using military, as part of a broad coalition, to go after them.

"We do so with a determination to ensure that there will never be a safe haven for terrorists who carry out these crimes," he added. "And we have demonstrated over more than a decade of relentless pursuit of al Qaeda, we will not be outlasted by extremists."

The President said Syria posed the greatest threat to international order. "When a dictator slaughters tens of thousands of his own people, that is not just a matter of one nation's internal affairs – it breeds human suffering on an order of magnitude that affects us all. Likewise, when a terrorist group beheads captives, slaughters the innocent and enslaves women, that's not a single nation's national security problem – that is an assault on all humanity."

He admitted that "compromise will be required" to end the war in the Middle East and eradicate ISIS, "but realism also requires a managed transition away from Assad and to a new leader, and an inclusive government that recognises there must be an end to this chaos so that the Syrian people can begin to rebuild."

The "poisonous ideology" of Islamic extremism "infects too many of our young people," Obama said, and Muslims must reject those who preach intolerance and promote violence, while non-Muslims must reject the worldview that equates Islam with terror.

The President also addressed issues of poverty, quoting Pope Francis in saying "we are stronger when we value the least among us, and see them as equal in dignity to ourselves and our sons and daughters". Other subjects noted in the wide-ranging address were climate change, Iran, and US relations with Cuba.

In a separate speech on Monday, Jordan's King Abdullah II branded the fight against ISIS "a third world war", and warned that the future is being threatened by the "outlaws of Islam that operate globally today".

"These outlaws gangs use suspicion and ignorance to expand their own power, and worse still, is the free hand they grant themselves to distort the word of God; to justify the most atrocious of crimes," he said.

"Let us amplify the voice of moderation. It is one of the greatest ironies of our times that extremist voices use advanced media to propagate ignorant ideas."

King Abdullah also called for the international community to respond to the Syrian refugee crisis. "It is the world's obligation to find solutions and provide relief for the millions of refugees in my region," he said.

According to the UN, Jordan has taken in more than 600,000 Syrian refugees.