(AP) -- 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 spoke as chairman of 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 on 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 not a conventional battle. 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 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.

--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.

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

On the central terrorism issue, leaders were dealing with an IS that attracting fighters from around the world, prompting fears they will return to their home countries to launch attacks.

The fight 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 speeches to the opening meeting of the annual U.N. General Assembly on Monday.

Obama reiterated Tuesday that defeating IS requires "a new (Syrian) leader and an inclusive government that united the Syrian people in the fight against terrorist groups."

Reflecting the divide, an official with the Russian delegation said Moscow would take part in the Obama-led event only by 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."

United Nations Secretary General Ban ki-Moon 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. He said security-focused counterterrorism measures are crucial but stressed the need to go beyond, including making a special effort to reach young people.

"Social media is central. 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," he said.

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."

"One terrorist fighting the other will not legitimize it," he said.

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

Iraq's Prime Minister Haider al-Abadi listed what he said were his government's successes fighting IS and recent anti-corruption drive. He 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.

Obama, in his speech, said Nigeria, Tunisia and Malaysia were the newest members of the U.S.-led international coalition. "We are harnessing all of our tools -- military, intelligence, economic, development and the strength of our communities."

Also set for Tuesday were high-level meetings on the humanitarian consequences of the war in Syria and the wider refugee and migrant crisis that is the largest since the upheaval of World War II. Refugees have swept into Europe in waves and already are jammed into camps in Jordan, Turkey and Lebanon.