U.S. President Barack Obama speaks as Iraqi Prime Minister Haider al-Abadi (R) smiles at the Leaders’ Summit on Countering ISIL and Violent Extremism at the United Nations General Assembly in New York.

U.S. President Barack Obama said Tuesday that defeating the Islamic State of Iraq and Syria (ISIS) group 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 U.N. 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 ISIS militants, who control large swathes of territory in Iraq and Syria.

Obama sparred with Russian President Vladimir Putin over the Syria crisis during their dueling U.N. 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.

Separately, Britain will countenance Assad staying in power for a transitional period but he cannot remain in charge long-term, Prime Minister David Cameron said on Tuesday.

“What America is saying, which I agree with, is that you need a transition but what’s clear about that is at the end of that Assad cannot be the head of Syria,” Cameron told CBS Television Network.

“It wouldn’t work because you would not be able to defeat ISIL (Islamic State) if that guy is still running the country,” Cameron said. “Assad is one of the recruiting sergeants for ISIL.”

“So far, the problem has been that Russia and Iran have not been able to contemplate the end state of Syria without Assad,” Cameron said, adding that he would work with any country to defeat ISIS militants.

After sending troops and fighter planes to Syria, Putin on Monday called for a “broad coalition” to defeat the militants 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 U.N. gathering when he vowed to crush ISIS and called on countries to join the United States in the campaign.

Taking stock of the campaign, Obama said ISIS 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.