The Iraqi city of Mosul will remain strewn with unexploded bombs for a decade, endangering a million or more civilians who want to return home following the end of three years of Islamic State occupation, a U.N. demining expert said on Wednesday.

Pehr Lodhammar, a senior program manager at the U.N. Mine Action Service (UNMAS), said the destruction of Mosul had left an estimated 11 million tons of debris and two-thirds of the explosive hazards were thought to be buried under the rubble.

"We estimate that it's going to be over a decade until western Mosul has been cleared. The density and the complexity will not allow this clearance to be completed within months or even within years," he told a news conference in Geneva.