British Prime Minister David Cameron will hold talks on Wednesday with Iran's President Hassan Rouhani over unrest in Iraq and Syria, officials said, announcing the first meeting between the countries' leaders since the 1979 Islamic Revolution.
The talks will occur on the sidelines of a UN General Assembly in New York. They come as Western powers seek to build support for the fight against the Islamic State (IS) jihadist group, which holds swathes of territory in Iraq and Syria and which has beheaded two US journalists and a British aid worker.
The United States and Arab allies launched the first air and sea strikes against IS militants in Syria on Tuesday, expanding action against the jihadists the US has been leading in Iraq since the start of August.
Although the United States excluded its longtime enemy Iran from the coalition it has built against IS, US Secretary of State John Kerry said on Friday that Iran still had a role to play in tackling the jihadists.
Iran, which usually rails against any US presence in the Middle East, has been unusually accepting of the US action in Iraq, where it is also tackling IS militants.
Rouhani has criticised the US for not sending in ground troops, and on Monday said the people in the region were "defending themselves... against the terrorists" and that Iran would help.
But Iran's supreme leader Ayatollah Ali Khamenei, whose regime staunchly supports the Syrian government, claims he rejected a US offer to join the international coalition against the IS.
Britain and France have taken up the task of trying to win some form of other cooperation from Iran against IS.
"If Iran is willing to join the international community to defeat ISIL (another name for IS), then we will work with them on that," a Downing Street source quoted by British media said.
French President Francois Hollande was also set to meet Rouhani on Tuesday.
France has joined the US in striking IS targets in Iraq, and Arab nations such as Saudi Arabia, Bahrain and Jordan joined in the aerial assault in Syria on Tuesday.
Britain, though, has so far not participated in the US-led military action against the IS group.
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Cameron has promised to ask for parliamentary approval for such involvement, but appears wary of a repeat of last year's humiliating defeat in the House of Commons over military action in Syria.
Britain's defence ministry said Tuesday that "discussions were ongoing" about possible action against IS.
Britain's Defence Secretary Michael Fallon was in Saudi Arabia and Bahrain Monday and Tuesday to discuss international action against the militants, officials said.
- 'Step by step' -
France has vowed it will continue its air strikes in Iraq despite the kidnapping of a Frenchman in Algeria claimed by an IS-linked group which threatened to murder him unless Paris desisted.
"It would be very odd in a way if UK air power was not part of this package," Professor Michael Clarke, director general of defence think-tank the Royal United Services Institute (RUSI), told BBC radio on Tuesday.
"Because Mr Cameron's very aware of the damage the Syria vote did just this time last year, he wants to make sure he gets it right this time so he's taking it step by step."
Britain is currently arming Kurdish peshmerga forces fighting IS militants in northern Iraq.
Britain has had tense ties with Iran since its revolution in 1979 brought anti-Western mullahs to power.
Amid an apparent thawing in relations recently, Britain said in June that it planned to reopen its Tehran embassy closed nearly three years ago when a government-sanctioned mob stormed it.