London, Sep 23 (IANS): British Prime Minister David Cameron is to hold the first bilateral talks with an Iranian president since the 1979 revolution when he meets President Hassan Rouhani in New York, where the annual UN General Assembly meet will take place.

The bilateral meeting, a significant thaw in diplomatic relations between the two countries, is designed to explore the support the Iranians can give in the fight against Islamic State (IS)militants, the Guardian reported Tuesday.

The Iranians were disallowed from attending the recent Paris talks on building an international coalition against IS and have also been excluded from all talks about dislodging Syrian President Bashar al-Assad, including the two conferences in Geneva.

Cameron is expected to make clear that IS (formerly called ISIS) is a common and extremely serious threat that can only be tackled in Iraq and that Iran must drop its support for President Bashar al-Assad.

He will also send a tough message to the Iranians on the nuclear negotiations that: "Iran has a rare opportunity to embolden its prosperity through a deal but this is only possible if Iran is willing to show flexibility and be realistic about the future scope of its nuclear programme, in particular the issue of enrichment."

"We are under no illusion about the dangers of Iran’s nuclear programme and our approach on that is not changing," sources in the British prime minister's office said.

"However, if Iran is willing to join the international community to defeat Isis then we will work with them on that, but will be clear you cannot take one approach in Baghdad and another in Damascus. You need a political solution in both if you are serious about defeating Isis."

Though a fierce critic of the IS, Rouhani has derided the US air strikes campaign as it involves no commitment to ground forces.