UNITED NATIONS — President Barack Obama on Tuesday pledged all possible tools — military, intelligence and economic — to defeat the Islamic State group, but acknowledged the extremist group has taken root in Syria and Iraq, is resilient and continues to expand.

Obama hosted a U.N. gathering of world leaders working to expand the battle against terrorism, a day after he and the leaders of Russia, China and Iran addressed the General Assembly during its 70th anniversary. The fight against terrorism, particularly in Syria, has seized the attention of top officials, but there has been no overall agreement on how to end the conflict there.

"I have repeatedly said that our approach will take time. This is not an easy task," Obama cautioned, while adding that he was "ultimately optimistic" the brutal organization would be defeated because it has nothing to offer but suffering and death.

"This is a long-term campaign — not only against this particular network, but against its ideology," he said.

The meeting also heard from the Iraqi leader, who sought more help against IS in his country, and learned from Obama that three more countries — Nigeria, Tunisia and Malaysia — were added to the coalition fighting the group.

In other events at the annual gathering of world leaders:

— Obama held talks with Cuban President Raul Castro, the second time the leaders of the once-estranged nations have met this year.

— The European Union's top diplomat emerged from a high-level meeting on Syria's humanitarian crisis to tell reporters that she sees possible "political space for new initiatives in the coming weeks," with no details.

— Ukraine's president condemned Russia's aggression against his country, and urged the international community to restrain Moscow's veto power in the U.N. Security Council.

— Guyana's president, David Granger, accused Venezuela of being a bully as the two countries brought their long-running border dispute to the world body.

— Japan said it is providing $1.5 billion for assistance of refugees and stabilization of communities facing upheaval in the Middle East and Africa.

— U.N. Secretary-General Ban Ki-moon scolded South Sudan's president, warning "not betray and disappoint us" in implementing a new peace deal.

The fight against terrorism has been complicated by a Russian military buildup in Syria in support of President Bashar Assad. Russian President Vladimir Putin suggested Monday that Russia could launch airstrikes against the militants in Syria, if sanctioned by the United Nations or requested by Damascus.

Obama and Putin are at odds over Russian involvement because Washington has said Assad must be removed from power. Obama and Putin laid out competing visions for Syria during their U.N. speeches Monday.

Reflecting the divide, an official with the Russian delegation said Moscow was taking part in the Obama-led event with a lower-level official, U.N. deputy ambassador Evgeny Zagaynov.

Russia on Wednesday will chair its own meeting on countering extremism as this month's U.N. Security Council president.

One by one, speakers at Tuesday's meeting spoke of the need to confront the extremism which Jordan's King Abdullah II described as the "greatest collective threat of our time."

Ban, the U.N. chief, said the U.N.'s most recent data shows a 70 percent increase in foreign terrorist fighters from over 100 countries to regions in conflict.

"Social media is central," he said, urging measures against online recruitment of young people. "We need to offer a counter-weight to the siren songs that promise adventure, but deliver horror — and that promise meaning, but create more misery."

Turkish Prime Minister Ahmet Davutoglu used the opportunity to highlight his government's fight against rebels of the Kurdistan Workers' Party, or PKK, saying "there is no difference between the Islamic State group and the PKK."

Kurdish fighters in northern Syria, however, have been one of the United States' most effective allies in the battle against IS.

Iraq's Prime Minister Haider al-Abadi appealed for the world's help in "drying up the sources of terrorism," including the flow of foreign fighters to Syria and Iraq, IS oil smuggling and money transfer networks.

More than 60 countries, including Arab nations, are working together and launching military airstrikes in an attempt to wipe out IS, which has taken control of large regions in Iraq and Syria.

Also set for Tuesday was a high-level meeting on the refugee and migrant crisis that is the largest since the upheaval of World War II.