“Islamic State has swept across Iraq, seized huge amounts of weapons and money and so it’s no surprise that insurgents inside Syria would seek their help,” said George Readings of the corporate intelligence company Stirling Assynt. Analysts believe that Islamic State is already making millions from the sale of oil from fields on its territory elsewhere and was the richest jihadist group in existence - assisted by stealing what was said to be more than £256 million from a branch of Iraq’s central bank in Mosul.