THOUSANDS of ISIS fighters are to be given safe passage out of Iraq to fight in Syria, Russian military leaders claim.

The jihadi army - and their families - will allegedly be let back in to Syria to battle president Bashar al-Assad and his Russian allies.

3 US President Barack Obama walks with Saudi King Salman Credit: Reuters

3 Islamic State fighters are feeling the heat in Iraq Credit: PA:Press Association Images

The outrageous claims comes as Britain and the US face criticism for failing to stop Assad’s brutal bombing of civilians in the Syrian city of Aleppo.

Nearly a thousand US and British troops have been sent to retake Mosul – the largest city under ISIS control.

The RAF and US Air Force are on standby to support any ground attack with major air strikes.

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But under the supposed agreement – allegedly brokered by Saudi Arabia – the US-led coalition will only bomb targets agreed with the militants in advance, reports the Daily Star.

The military source in Moscow told the Star: "In preparation for the operation in Mosul, US intelligence agencies and Saudi Arabia agreed that before the assault all militants will be offered a safe route to leave the city with their families.

Tensions with Russia have reached levels not seen since the Cold War. Vladimir Putin has rattled the US with preparations for nuclear war, including an evacuation drill for 40million people, building secret nuclear bunkers for top brass and ordering politicians families back to 'the motherland'.

3 Credit: Alamy

“And at the time of the assault, coalition aircraft would only strike empty buildings, agreed in advance with the militants.

"More than 9,000 ISIS fighters will be transferred to the eastern regions of Syria to follow a major offensive operation, which involves the capture of Deir ez-Zur and Palmyra.”

Saudi Arabia has already been accused of supplying funds and arms to ISIS and other Islamic rebel groups fighting Assad.

Vladimir Putin has thrown his military might behind pal Assad as long-time ally Syria has hosted Russian bases since the Soviet era – and gives Russia key access to the Mediterranean.