London, UK (BBN)-David Cameron will call for a new diplomatic drive to end Syria's civil war as he meets world leaders at the United Nations in New York on Sunday.
The UK prime minister is expected to drop his opposition to Syrian President Bashar al-Assad playing a role in any transitional government, reports BBC.
But he is expected to insist Assad stands down to allow reconciliation.
US Secretary of State John Kerry said he hopes the talks can help find a political solution to Syria's crisis.
Russian President Vladimir Putin and US President Barack Obama will be among other leaders attending the 70th anniversary meeting of the UN General Assembly.
Cameron is expected to use a series of meetings with other leaders - including Obama - to argue that a peaceful solution will require a different leadership which the whole country can unite around.
"Our view is very clear, Isil (Islamic State) and Assad are both the enemies of the Syrian people," one senior British official told the Press Association.
"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."
UK officials also warned finding a solution to the four-year conflict had been made more complicated by Russian military build-up in support of Assad's regime.
Russia is reported to have moved warplanes, helicopters, tanks, drones, artillery and troops into a military airbase near Latakia and the seaport of Tartus in recent weeks.
Cameron is not expected to meet Putin.
But British officials have acknowledged they will need to work with Russia to find a diplomatic solution.
'Complex and challenging'
"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. That is where there is more discussion ongoing," a senior British official told PA.
"There does need to be more thinking in capitals on what it would be," the official said.
"This is complex and challenging and at the moment we are still trying to work out and agree what that solution might be."
Cameron is also expected to urge other leaders to do more to support the millions of refugees still in the region.
France's foreign minister, meanwhile, told the General Assembly that President Assad should not play a role in Syria's future.
Laurent Fabius said Western countries would be exposing themselves to failure if they told Syrians their future lay with Assad.
On Thursday, German Chancellor Angela Merkel said Assad should be involved in peace talks, but Fabius urged a united European strategy against such involvement.
Kerry said he had discussed the crisis with Iran's foreign minister and would do the same with Russian representatives on Sunday.
BBN/SK/AD