BEIRUT: Isil militants have rigged one of Iraq's largest chemical complexes with explosives as they prepare to defend their stronghold of Mosul from advancing forces.
US troops and the Iraqi army expect Islamic State of Iraq and the Levant to put up a fierce fight for the city of 1.2 million people - the largest under its control.
Colonel Hamish de Bretton-Gordon, a former commanding officer of Britain's Joint Chemical, Biological, Radiological and Nuclear Regiment (CBRN) said commanders had intelligence that the jihadists were planning to blow up the plant when soldiers approached.
He has been advising Kurdish Peshmerga troops fighting Isil east of the city.
Misraq chemical plant and sulphur mine lies 30 miles south of Mosul and six miles north of Qayyarah airbase, where several hundred US troops are stationed alongside the Iraqi army.
An explosion at Misraq, which is thought to hold thousands of tons of sulphur and hydrogen sulphide, would be a major concern as ground forces advance.
According to calculations by Col de Bretton-Gordon, the fallout could have a radius of six to 10 miles, putting Iraqi, and any supporting US, forces at risk.
Burning pure sulphur produces corrosive and toxic sulphur dioxide. A fire at the plant in 2003 burned for a month and released half a million tons of sulphur dioxide into the air, damaging the ozone layer, leaving hundreds of people in hospital with breathing problems and killing vegetation for miles.
It is described as one of the worst man-made disasters in history.
"They're going to throw the kitchen sink at any army that comes near Mosul," said Col de Bretton Gordon. "It'll be catastrophic."
Haider al-Abadi, Iraq's prime minister, said this week that the army was preparing to begin the battle for Iraq's second city, held by Isil since 2014, next month.
In a sign of what could be to come, the jihadists fired a rocket containing suspected mustard gas at the base in Qayyarah on Tuesday, their first chemical attack on US servicemen in Iraq.
US officials said the soldiers were as yet not suffering any symptoms.
Initial tests showed a mustard agent, a banned chemical that can burn the skin and lungs. It has been sent to a lab for definitive analysis.
"It was mustard agent in a powderised form - the same thing we have seen [Isil] use to little effect many times in the past in both Syria and Iraq," said a Pentagon spokesman, adding that the agent was "low-grade" and poorly weaponised. "The device, likely a rocket or mortar, was imprecise and crude."
Russia yesterday (Thursday) rejected US proposals to halt military flights over Syria, saying fragile ceasefire could only be restored if Washington pressured rebel groups.