British Prime Minister David Cameron will tell the UN that peace in Syria is impossible while President Bashar al-Assad remains in power, officials say.
Cameron will to fly out to the United Nations in New York on Sunday (local time) as Western diplomats scramble to cobble together a diplomatic strategy to end the civil war in Syria.
British officials say attempts to resolve the four-year conflict have been made more difficult by Russia's recent military support for the Assad regime.
Russian President Vladimir Putin, US President Barack Obama and Chinese President Xi Jinping are set to attend the 70th anniversary meeting of the UN General Assembly.
Moscow has recently deployed troops and warplanes to Syria, combined with new arms deliveries to Assad's forces taking on the Islamic State jihadist group that now controls parts of Syria and Iraq.
"Our view is very clear: ISIL and Assad are both the enemies of the Syrian people," a senior British official said, on condition of anonymity.
"The prime minister's view is still very clearly that in the endgame you need a different leader to build a peaceful and inclusive Syria."
US Secretary of State John Kerry and his European counterparts reached out to traditional foe Iran on Saturday on the sidelines of the UN gathering.
Iran, like Russia, backs Assad, who is seen by Washington as the instigator of the civil war that has left half of Syria in Islamic State's hands.
Cameron will use a series of one-to-one meetings with other leaders, including Obama, to press the case that a peaceful solution would ultimately require different leadership in Syria around which the majority can unite.
However, Britain has stressed that Assad would not necessarily have to go immediately as part of any peace deal.
"There has always been the idea that there will be a political transition and there are differing views between members of the international community... what the steps are in the process," a senior British official said.
"That is where there is more discussion ongoing."
Cameron was to stress that the migrant crisis in Europe could spur world powers into a new push to end the conflict in Syria.