President Barack Obama sits down with more than 100 leaders at the United Nations on Tuesday to push ahead with the US-led campaign against the Islamic State group despite Russia's rival plan.

Russia has been invited to the counter-terrorism summit held a day before Moscow hosts a special UN Security Council meeting on the same issue, two events bound to highlight sharp differences in approach.

The meeting comes a day after Obama clashed with President Vladimir Putin over the crisis in Syria during duelling UN speeches, but said the United States was willing to work with Russia and Iran to end the four-year conflict.

After sending troops and fighter planes to Syria, Putin called for a "broad coalition" to defeat the jihadists and warned it would be an "enormous mistake" to sideline President Bashar al-Assad's military from the fight.

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

Since then, the jihadists 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's Boko Haram.

Iraqi leader Haider al-Abadi and Nigerian President Muhammadu Buhari are among the key speakers at the event, held on the sidelines of the General Assembly.

Norway's Prime Minister Erna Solberg, whose country was badly shaken by the 2011 massacre of 77 people by right-wing extremist Anders Brieivik, will also address the gathering.

One country that has not been invited is Iran even though it is playing a major role in the fight against IS in Syria and Iraq, providing military advisers, weapons and trainers.

In his address to the General Assembly, Iranian President Hassan Rouhani called for a "united front" against the extremists and like Russia, argued that the Syrian regime must take part.

"The gravest and most important threat to the world today is for terrorist organizations to become terrorist states," said Rouhani.

- Taking stock a year on -

The 104 leaders will discus 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 IS, US officials told The New York Times at the weekend.

The US-led coalition that now comprises some 60 countries including Syria's neighbors has carried out more than 5,000 air raids, pounding IS 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.

Washington has insisted that Assad must leave power as a pre-requisite for any settlement to the conflict, while European powers have softened their stance, signalling he could stay on in an interim role.

On Wednesday, Russian Foreign Minister Sergei Lavrov hosts a Security Council meeting on counter-terrorism expected to again fuel debate on the way forward in Syria.

Russia had circulated a draft statement to the 15 council members that it hoped could have been adopted at the meeting, but the United States balked at language linking the fight against terrorism with "affected states".

The statement "could be perceived as endorsing an approach that could set back efforts to reach a negotiated political transition in Syria," a US State Department official said.

Obama says defeating IS 'requires a new leader' in Syria
United Nations, United States (AFP) Sept 29, 2015 - US President Barack Obama said Tuesday that defeating the Islamic State group in Syria will only be possible if Bashar al-Assad leaves power, a day after a clash with Russia over the Syrian president's fate.

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

Russia snubbed the summit called by the United States, sending a low-level diplomat to the meeting to take stock of the one-year campaign to defeat IS jihadists, who control large swathes of territory in Iraq and Syria.

Obama sparred with Russian President Vladimir Putin over the Syria crisis during their dueling UN speeches on Monday, but the leaders agreed to work together to try to end the four-year war that has killed more than 240,000 people.

Assad's fate is the key bone of contention between Washington and the Syrian leader's Russian and Iranian allies.

After sending troops and fighter planes to Syria, Putin on Monday called for a "broad coalition" to defeat the jihadists and warned it would be an "enormous mistake" to sideline Assad's military from the fight.

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.

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

Taking stock of the campaign, Obama said IS had lost a third of the territory 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.