By Dominic Waghorn, US Correspondent
David Cameron is to become the first British Prime Minister to meet an Iranian President since 1979 in a historic encounter in New York.
Downing Street says the meeting with Hassan Rouhani is part of efforts to mobilise support against the Islamic State and drop Iran's support for the Assad regime in Syria.
The Prime Minister will be attending the United Nations general assembly and is hoping to secure UN approval for a comprehensive strategy to deal with the brutal militant group.The PM will dicuss the threat posed by Islamic State with Iran's president
It marks Mr Cameron's determination to enlist the active support of regional powers in the Middle East in taking on IS, which has seized control of swathes of territory in Iraq and Syria.
The group has also carried out a string of atrocities against local people and foreigners as it seeks to impose its own brand of extremist Sunni Islam.
Mr Cameron will also join US President Barack Obama and other members of the UN Security Council to discuss the threat posed by foreign fighters - thought to include hundreds of Britons - engaged in the conflict.
Officials say he will set out in his speech that the threat from extremism is faced by everyone and international action is required to defeat it.
World leaders meet annually for the event in New York which is often criticised for being long on talk and short on action.
Ahead of the encounter, Iran's supreme leader Ayatollah Khamenei issued damning criticism of the UK and the US calling them warmongers for their planned actions in Iraq.Also on the agenda with be Iran's nuclear-enrichment programme
Britain has yet to commit support for air strikes and there has been US speculation it may step up its military intervention now the Scottish independence referendum is out of the way.
Britain and the US also hope to see a new UN Security Council resolution preventing the flow of foreign fighters into Iraq and Syria.
Tehran will also call for flexibility on its uranium-enrichment programme which has led to sanctions due to Western concerns any nuclear capability could be used militarily.
Downing Street said Mr Cameron had "no illusion" about the threat posed by Iran's nuclear operations, but was ready to work with Tehran if it was willing to join the effort to defeat IS.
There is a sense of urgency this year given the threat posed by IS, but also the catastrophic dangers posed by global warming.
Ahead of the meeting UN Secretary General Ban Ki-moon warned time is running out to unite against the global threat of climate change.