ISIS terrorists blew up barrels of crude oil at the top of the yellow sulphur mountain.
A fire lit by the ISIS group at a sulphur plant south of the Iraqi city of Mosul was brought fully under control on Thursday, officials said.
The fire set off by jihadists last week at the Mishraq plant released huge clouds of toxic smoke that caused hundreds to seek medical assistance and killed at least two civilians.
Two hundred men from the federal police and civil defence fought the fire for a week at the huge quarry, civil defence chief General Kadhem Salman told AFP.
"The fire is under control, but 4.5 square kilometres (1.7 square miles) have burned," he said.
Iraqi forces launched a massive offensive 10 days ago on Mosul, the country's second city and the last major stronghold of the jihadists in Iraq.
ISIS fighters have used obfuscating tactics -- setting oil wells, tyres and in this case a sulphur plant on fire -- to provide cover from the aerial assets of the Iraqi forces and their allies.
The task of putting out the fire at the Mishraq plant was made difficult by "winds that changed directions sometimes seven times a day" and the nature of the soil, composed of highly pure sulphur.
When ISIS terrorists blew up barrels of crude oil at the top of the yellow sulphur mountain, a total of 1.5 million tonnes could have been set ablaze, Luay Jawad, who heads the federal police's engineering corps, told AFP.
The emergency teams dug a 2.5-metre (eight-foot) deep trench they filled with fire-extinguishing foam to prevent the fire from spreading, he said.
The United Nations said Iraq's health directorate treated more than 1,000 cases of suffocation in several locations south of Mosul, mostly in nearby Qayyarah.
It also said a water plant was affected by fighting that caused a chlorine leak for which 100 civilians sought treatment.
The UN environment agency condemned "what appears to be a scorched-earth policy employed by retreating" ISIS terrorists.
"This is sadly just the latest episode in what has been the wholesale destruction of Iraq's environment over several decades," said UNEP chief Erik Solheim in a statement.
"This ongoing ecocide is a recipe for a prolonged disaster. It makes living conditions dangerous and miserable, if not impossible. It will push countless people to join the unprecedented global refugee population," he said.
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