US President Barack Obama has said that defeating the so-called Islamic State group in Syria will only be possible if Bashar al-Assad leaves power.
"In Syria, defeating ISIL [IS] requires a new leader," Mr Obama told a counter-terrorism summit of about 100 leaders, held on the sidelines of the UN General Assembly.
Mr Obama yesterday clashed with Russia over the Syrian president’s fate.
Russia snubbed today's summit called by the United States, sending a low-level diplomat to the meeting.
The meeting is taking stock of the one-year campaign to defeat IS, which controls large swathes of territory in Iraq and Syria.
Mr Obama sparred with Russian President Vladimir Putin over the Syria crisis during their UN speeches yesterday, but the leaders agreed to work together to try to end the four-year war that has killed more than 240,000 people.
Mr 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, Mr Putin yesterday called for a "broad coalition" to defeat IS and warned it would be an "enormous mistake" to sideline Mr Assad's military from the fight.
Mr 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 Mr Assad must leave power, but Mr 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 Mr Obama vowed at the last UN gathering to crush IS and called on countries to join the United States in the campaign.
Taking stock of the campaign, Mr 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.
However, he added that military action alone would not succeed and that the coalition must address the conditions that allow Islamic radicalism to thrive.
Russia later criticised the holding of the counter-terrorism summit, saying it was disrespectful.
"This initiative seriously undermines UN efforts in this direction," Russia's UN envoy Vitaly Churkin was quoted as saying by Russian news agencies.
"The UN has its own anti-terror strategy and everything could easily be done within the UN framework.
"But Americans would not be Americans if they did not seek to demonstrate their leadership," Mr Churkin said.
