Isis militants in Iraq plan to invade Iran to seize details of its nuclear programme and give it to Moscow, in exchange for help defeating Syrian president Bashar al-Assad, the Sunday Times claims.

According to newspaper, an Isis (Islamic State) manifesto, believed to be written by its commander Abdullah Ahmed al-Meshedani, states the militant group is planning to take its battle to Iran, to obtain Tehran's nuclear secrets.

The papers were seized by Iraqi authorities during a counter-offensive against Isis, and were verified by Western intelligence authorities. It was thought to have been seized during a raid on a senior militant's house.

In return for giving Russia access to oil and gas fields in the Anbar province of Iraq, which is controlled by Islamic State, the militant group was to ask Moscow to stop supporting the Syrian president and Iran's Shi'ite-controlled government, and instead support Sunni-controlled governments in the Middle East and give Islamic State control over Iran's nuclear programme.

The Sunni insurgent group's manifesto also reportedly contained plans to uproot the Shi'ite-dominated Iranian government in order to spread its self-proclaimed caliphate, and plans to ethnically cleanse the Middle East of Shi'ite Muslims.