US President Barack Obama said Tuesday that Syrian President Bashar al-Assad must go if the Islamic State group is to be defeated, as he rallied world leaders to reinvigorate the coalition campaign against the jihadists.

A day after clashing with Russian President Vladimir Putin over how to handle the crisis in Syria, Obama hosted a counter-terrorism summit at the United Nations to take stock of the one-year air war against IS fighters in Iraq and Syria.

"In Syria (...) defeating ISIL requires, I believe, a new leader," Obama told the gathering of some 100 leaders, held on the sidelines of the UN General Assembly.

Russia snubbed the meeting, sending a low-level diplomat after Putin stole the limelight with his UN speech calling for a broad coalition to fight ISIS that would include Syria's army.

Assad's fate is the key bone of contention between Washington and the Syrian leader's Russian and Iranian allies amid intense diplomacy over the way forward to end the four-year war that has killed more than 240,000 people.

At the summit, Obama said the United States was ready to work with Russia and Iran to "find a political mechanism in which it is possible to begin a transition process."

The United States has long insisted that Assad must leave power, but Obama did not specify in his remarks whether the Syrian leader could take part in a transition in an interim role.

Russia's UN Ambassador Vitaly Churkin blasted the US-led summit as "disrespectful" toward the United Nations, saying it "seriously undermines UN efforts in this direction."

On Wednesday, Russia is to host a special UN Security Council meeting on the same issue -- an event bound to highlight sharp differences in approach.

French Foreign Minister Laurent Fabius accused Russia of displaying bravado on the Syria crisis that had yet to be backed up with action against the ISIS group.

"You have to look at who is doing what. The international community is striking Daesh. France is striking Daesh. The Russians, for the time being, are not at all," Fabius told a news conference, using the Arabic acronym for ISIS.

"If one is against the terrorists, it is not abnormal to strike the terrorists," Fabius added.

The counter-terrorism summit takes place a year after Obama stole the limelight at the last UN gathering when he vowed to crush ISIS and called on countries to join the United States in the campaign.

Taking stock one year on, Obama said IS had lost a third of the "populated areas" it controlled in Iraq and had been "cut off" from almost all of Turkey's border region.

But he added that military action alone would not succeed and that the coalition must address the conditions that allow Islamic radicalism to thrive.

Iraqi Prime Minister Haider al-Abadi called for international aid to equip his troops fighting the jihadists, who triggered alarm after seizing the city of Mosul in June last year.

Since then, ISIS fighters have captured territory in Syria and Iraq and gained a foothold in Libya, Yemen and elsewhere in the Middle East, with alliances as far afield as Nigeria, with Boko Haram.

Iran was not invited to the summit even though it is playing a major role in the fight against ISIS, providing military advisers, weapons and trainers.

The 104 leaders discussed combating foreign fighters and countering violent extremism as reports show the flow of jihadists to Iraq and Syria has continued unabated.

US intelligence fears nearly 30,000 foreign fighters have traveled to Iraq and Syria since 2011, many of them to join ISIS, a task force said in a key report released in Congress on Tuesday.

The US-led coalition that now comprises some 60 countries including Syria's neighbors has carried out more than 5,000 air raids, pounding ISIS targets in Iraq and Syria, with France this week joining the campaign in Syria.

Aside from the aerial bombardment of IS targets, the Pentagon has set up a $500 million program to train "moderate" Syrian rebels.

But that tactic has turned into a fiasco after the Pentagon said only a few dozen of fighters had been trained and that some of those had handed over their weapons to Al-Qaeda's affiliate in Syria.