US President Barack Obama says that Syrian leader Bashar al-Assad must go if the Islamic State group is to be defeated, as he rallied world leaders to revitalise 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 counterterrorism 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, held on the sidelines of the UN General Assembly.
Russia snubbed the meeting of some 100 leaders, sending a low-level diplomat after Putin stole the limelight with his UN speech calling for a broad coalition to fight IS that would include Syria's army.
Assad's fate is the key bone of contention between Washington and Syria'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.
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 whether the Syrian leader could take part in a transition in an interim role.
Easing Assad out of office through a two-stage transition is one of the options being discussed between US, Russian and Gulf diplomats.
On Wednesday, Russia is due to host a special UN Security Council meeting on confronting terrorist threats - an event bound to highlight sharp differences in approach.
Hinting at a possible compromise, US Secretary of State John Kerry said Washington could co-operate on Syria if Russia and Iran persuade Assad to stop using barrel bombs against civilians.
"They are both in a position, in exchange perhaps for something that we might do, they might decide to keep Assad from dropping barrel bombs," Kerry said in an interview with MSNBC.
Western diplomats maintain that Assad has killed more civilians by using barrel bombs dropped from helicopters than IS in its brutal advance in Syria.
Saudi Foreign Minister Adel al-Jubeir echoed Obama's call, saying the Syrian leader must step down or face the prospect of being removed by force.
"There is no future for Assad in Syria, with all due respect to the Russians or anyone else," Jubeir told reporters.
Taking a swipe at Russia, French Foreign Minister Laurent Fabius accused Moscow of displaying bravado on the Syria crisis that had yet to be backed up with action against the fighters.