The US and Saudi Arabia have agreed to grant free passage to thousands of Islamic State militants before the Iraqi city of Mosul is stormed. The jihadists will be redeployed to fight against the government in Syria, a military-diplomatic source told RIA Novosti.

"More than 9,000 Islamic State (IS, formerly ISIS, ISIL) militants will be redeployed from Mosul to the eastern regions of Syria to carry out a major offensive operation, which involves capturing Deir ez-Zor and Palmyra,” the source said.

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According to the anonymous diplomatic source, US President Barack Obama has already sanctioned an operation to liberate Mosul, due to take place in October.

During the storm of the city in northern Iraq the US-led coalition’s planes would only strike detached, vacated or uninhabited buildings, while keeping terrorists as targets, he said.

In September, US Secretary of Defense Ashton Carter confirmed that Washington would send an additional 600 troops to Iraq to help liberate Mosul at the request of the local authorities.

The source suggested that redeployment of IS militants is necessary because “Washington must somehow counter Russia’s achievements in Syria, try to diminish their importance.”

"Apart from the purely political dividends, the other purpose of this operation, obviously, will be to discredit the success of Russian Airspace forces. And, of course, it’s an attempt to undermine Syrian President (Bashar) Assad,” he said.

The leadership of Saudi Arabia’s General Intelligence Directorate will be the mediators and guarantors of the agreement on safe passage for the jihadists from Mosul, he claimed.

The source added that a similar scheme had been used by the US and its allies during the liberation of the Iraqi city of Fallujah.

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Damascus has accused Washington for coordinating with IS after an airstrike against the Syrian government troops near the city of Deir ez-Zor on September 17. Washington said that the bombing, in which 83 soldiers were killed and over 100 injured, was a mistake.