Ayatollah Ali Khamnei authorised his top commander to coordinate with US forces, it was claimed

The mission to crush the Islamic State out of existence leaves the US and the UK facing the unpalatable prospect of having to make common cause with foes in Iran and Syria.

Iran's Supreme Leader, Ayatollah Ali Khamenei, has approved contacts with US military officials to help defeat jihadists, it was claimed last night.

Ayatollah Khamenei has authorised his top commander to co-ordinate military operations with the US, Iraqi and Kurdish forces, sources in Tehran told the BBC.

Iran - which has called the US 'the Great Satan' - has traditionally opposed US involvement in Iraq, an Iranian ally.

Iran's foreign ministry officially denied it planned to cooperate with the US, but Iraq's president Fuad Masum was said to have confirmed the extraordinary development.

Shia Iran sees the extremist Sunni Islamic State group as a grave threat.

The former chief of the defence staff, General the Lord Richards, said Britain should ask Iran and Syria for assistance - and also persuade Russia to step in, despite the conflict in Ukraine.

Asked if the West should consider a deal with the Syrian regime, which it was proposing to attack a year ago, Lord Richards said: 'My judgement is that you do have to come to some accommodation with them.

'Russia, ironically could play a very important role in that and Iran too.

'There are the bones for a grand strategic solution to the Middle East here, if we can get together with people who we viewed as rather hostile.

'This is happening all over the world and needs to be the first stage in a grand strategic plan to once and for all deal with Muslim extremism in places such as Nigeria, Kenya, Somalia, Yemen, Pakistan, and now we hear in Myanmar.'

David Cameron dismissed the idea that the Government should co-operate with President Bashar Assad in Syria to tackle the Islamic State - insisting he was 'part of the problem'.

The alliance would bring together previously hostile governments in a bid to overthrow extremist Muslims in the Middle East

General Lord Richard Dannatt, former head of the Army, said Britain should ask Iran and Syria for assistance in overcoming ISIS

His suggestion for Britain to appeal to President Assad (right) was dismissed by David Cameron who described the Syrian leader as 'part of the problem'

Assad's 'brutal' regime had encouraged the rise of Isis by persecuting parts of Syria's population.

The Prime Minister said he still wanted to see a "transition" of regime in that country.

'Just as we need in Iraq a democratic pluralistic government to represent all of that country -Sunni and Shia and Kurd - so in Syria we need a transition to a government that can represent all the people in that country and stop brutalising them,' he said.