UNITED NATIONS -- President Barack Obama said Tuesday he's well aware of the difficult fight ahead to defeat the Islamic State and other extremists but urged world leaders to go after the terrorist groups with renewed vigor.

"This is not an easy task," Obama told members of the Leaders' Summit on Countering ISIL and Violent Extremism, a coalition he formed to take on terrorist groups.

Obama's remarks came the same day a House committee accused his administration of failing to stop Americans from joining the Islamic State group, also known as ISIL or ISIS. Russia, which differs sharply with the United States on how to end Syria's war with ISIS and other terrorist groups, is set to preside over a meeting today on the topic.

The United States also Tuesday announced it was imposing sanctions on 35 individuals it suspects of supporting terrorism, including members of ISIS.

The president said ISIS has succeeded by attracting recruits worldwide through the Internet and establishing a foothold in unstable nations.

"We have ISIL taking root in areas that already are suffering from failed governance, in some cases, civil war or sectarian strife," Obama said. "They have shown themselves to be resilient, and they are very effective through social media and have been able to attract adherents not just from the areas in which they operate, but in many of our own countries."

Both Russia and the United States agree that Islamic State poses the world's greatest terrorism threat, but the nations' leaders are opposed on the question of Syria's leadership once its civil war ends.

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Russian Foreign Minister Sergey Lavrov is set to run a Security Council meeting on counterterrorism.

Obama reiterated Tuesday that Bashar Assad must not lead the postwar Syria, saying his violent crackdown on dissent in March 2011 fueled terrorism by magnifying the grievances some of his people and their supporters had against him.

Syria then became embroiled in a civil war and a magnet for foreign terrorist fighters as Islamic State and Nusra Front amassed armies to fight.

UN Secretary-General Ban Ki-moon said the UN, which has adopted measures including the Global Counter-Terrorism Strategy and Security Council Resolution 2178, is uniquely positioned to fight terrorism with a unified response.

"Violent extremist groups -- including Daesh and Boko Haram -- pose a direct threat to international security, mercilessly target women and girls, and undermine universal values of peace, justice and human dignity," he said, using the Arabic acronym for ISIS.

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Obama's claim that Assad must go came a day after Russian President Vladimir Putin said Assad is the legitimate leader of Syria and should not be removed from power.

Obama's counterterrorism summit drew heads of state and government from countries where ISIS or its adherents have struck -- including Iraq, where ISIS has a stronghold, a large swath of Syria, and Nigeria where Boko Haram, another terrorist group, has attacked civilians in African nation's northeastern Borno state.

Boko Haram drew international scorn in April 2014 when it abducted nearly 300 schoolgirls from Chibok, most of whom are still missing.

The summit came as a House of Representatives Homeland Security Committee released a report criticizing the Obama administration for failing to prevent Americans from leaving the United States to join ISIS and then returning. "The U.S. government lacks a national strategy for combating terrorist travel and has not produced one in nearly a decade," was one of the 32 findings in the report, "Combating Terrorism and Foreign Fighter Travel," which estimated that 250 Americans have tried to join ISIS.

Experts estimate that up to 30,000 fighters have traveled to Syria to join Islamic State, many of them men and women from Western nations lured by the Sunni Islam group's extreme ideology and demonstrated military prowess.

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ISIS has established what it calls a caliphate on the border between Iraq and Syria by seizing key cities in both countries while waging battle against the Shia-affiliated government of Bashar Assad. Human rights groups have documented barbaric practices carried out by ISIS against populations it captures, such as the Yazidis in northern Iraq, whose women and girls have fallen victim to systemic rape and sexual slavery while its men and boys have been slaughtered.

ISIS, which took advantage of the civil war in Syria that began on March 2011 in the wake of the Arab Spring - a popular movement that spread across the Middle East and toppled several strongmen - has shocked the world with grisly viral videos of captives being beheaded or burned alive, including Western journalists and aid workers.

Obama announced on Tuesday that there are now 60 or so countries aligned against Islamic State, with Tunisia, Malaysia and Nigeria among the most recent recruits. The summit's targets also include Boko Haram, Al Shabab, Al Qaeda and its affiliate in Syria and Iraq, Al Nusra Front.

"There are going to be successes and there are going to be setbacks," he continued. "This is not a conventional battle. This is a long-term campaign -- not only against this particular network, but against its ideology."