Two Iraqi army pilots were killed on Thursday when their helicopter was shot down over the city of Mosul by Islamic State, according to a military statement.The helicopter was providing air support to Federal Police forces battling Islamic State fighters on the western side of Mosul, the statement said.It is the first aircraft downed by Islamic State over Mosul since the start of the U.S.-backed offensive on the northern Iraqi city, in October.The insurgents are putting up stiff resistance in the remaining district under their control in northwestern Mosul and the densely populated Old City.The militants are dug in surrounded by civilians, effectively using them as human shields and taking advantage of the narrow streets of the Old City that restrict the movements of the Iraqi forces and limit the use of artillery and air power.