President Obama -- accompanied by, from left, National Security Adviser Susan Rice, Secretary of State John F. Kerry, and U.S. Ambassador to the United Nations Samantha Powe -- listens as Iraqi Prime Minister Haider al-Abadi, speaks at Tuesday's summit at the United Nations. (Andrew Harnik/AP)

This story has been updated.

UNITED NATIONS — President Obama convened a summit at the United Nations Tuesday aimed at countering violent extremism, arguing the world will ultimately beat back Islamist militants with "better ideas."

Speaking to representatives of 100 nations, more than 20 multilateral institutions and private-sector officials, Obama thanked them for "answering the call" to action, and hailed “the emergence of a global movement” to take on the Islamic State, Boko Haram and other extremists. In many ways, the session represented a reprisal of what world leaders have said before on the subject.

“Our approach will take time. This is not an easy task,” he said, adding that the international community needs to be "clear-eyed" about the challenge it faces.

"This is not going to be turned around overnight, because it is not just a military campaign that we are involved in," the president said, adding that many of the problems that have fostered militancy "have built over decades."

The president focused most of his comments on the Islamic State, also known by the acronym ISIL, saying the practices of Syrian President Bashar al-Assad has given the terrorists an opening.

"We have ISIL taking root in areas that already are suffering from failed governance in some cases -- in some cases, civil war or sectarian strife," Obama said. "And as a consequence of the vacuum that exists in many of these areas, ISIL has been able to dig in. They have shown themselves to be resilient, and they are very effective through social media."

[Obama and Putin outline competing visions on Syria]

But Obama emphasized that the fight against the Islamic State "it is not going to won the battlefield."

"This means defeating its ideology," he said. "Ideologies are not defeated with guns, they're defeated by better ideas."

Leaders from two of the countries facing the most serious terrorist threat — Iraq and Nigeria — sat at the head table, along with Obama, U.N. Secretary General Ban Ki-moon and Norwegian Prime Minister Erna Solberg.

Iraq Prime Minister Haider al-Abadi said his country was determined to push back against the Islamic State, "which is not only a threat to us but to the whole world."

[Iraq turns to Russia in fight against Islamic State]

Abadi began several sentences with the phrase, "we need your help," declaring, "We need your help, and the help of the international community."

He bemoaned the fact that his government was engaged in a fight with "radicals who are coming from all places in the world," including North America.

And Nigeria President Muhammadu Buhari said that even if the final victory might not come from armed force, his country still needs military aid to combat Boko Haram and other militants.

"Guns alone may not suffice, but they can stem the tide" against terrorists, Buhari said.

[Why Nigeria’s president thinks the U.S. has ‘aided and abetted’ Boko Haram]