President Obama told more than 100 world leaders Tuesday that defeating the Islamic State will require "diligence, focus and sustained efforts by all of us" at the second summit on combating violent extremism during the United Nations General Assembly.

"I challenged countries to return to the General Assembly this year with concrete steps that we can take together" to defeat the Islamic State, also known as ISIS or ISIL. Obama, flanked by U.N. Secretary General Ban Ki-Moon and Iraqi Prime Minister Haider al-Abadi, thanked the leaders "for answering this call."

He noted that Nigeria, Tunisia and Malaysia recently joined the international coalition confronting the Islamic State.

Obama said he is seeing "the emergence of a global movement that is united by the mission of degrading, and ultimately destroying, ISIL." The 60-country coalition fighting the Islamic State in Iraq and Syria is "harnessing all of our tools, military, intelligence, economic, and the strength of our communities," to combat "the violent extremism that ISIL needs to survive," Obama said.

Obama conceded that in Iraq, the Islamic State still holds the key cities of Mosul, Fallujah and Ramadi. But he said Iraqi forces, aided by the U.S.-led coalition, have reclaimed one-third of former Islamic State territory.

"They have shown themselves resilient," Obama said of the terrorist group. "They have been able to attract adherents … in many of our own countries."

"There are going to be successes and there are going to be setbacks; this is not a conventional battle," he added. "This is a long-term campaign … against its ideology."

"It is not going to be enough to defeat ISIL in the battlefield," Obama continued. The coalition will "have to defeat their ideology; ideologies are not defeated by guns but by better ideas," he said, calling on the international community to help Abadi create the political and economic opportunities necessary to keep the Islamic State from winning followers.

Groups such as the Islamic State and Boko Haram in Africa "exploit" political and economic vacuums and the grievances they create to win acolytes, Obama said. Young people in particular need jobs and their countries need sustainable development to keep another Islamic State from taking root, he said.

"Even if we were to wipe out the entire cadre of ISIL leadership, we would still have some of these forces at work," he said. "Defeating ISIL [in Syria] will require, I believe, a new leader," Obama said.

But ultimately Obama said he is "optimistic" that international cooperation, good governance, sustainable development and effective anti-propaganda campaigns can cripple the Islamic State.

"Like terrorists and tyrants throughout history, ISIL will eventually lose because it has nothing to offer but suffering and death," Obama said, adding that the life the Islamic State offers "is a stark and brutal life that does not appeal to people over the long term."