ERBIL, Iraq, Feb 18 - Iraq's US-backed offensive against Islamic State in western Mosul could displace up to 400,000 civilians and involve a siege in the densely populated old city, the United Nations' Humanitarian Coordinator for Iraq said on Saturday.The military and security forces last month recaptured the eastern half of the city, Iraq's second largest, and are expected to advance on the western side in coming days.Western Mosul contains the old city center, with its ancient souks, Grand Mosque and most government administrative buildings. Commanders expect the battle to be more difficult than in the east because, among other things, tanks and armored vehicles cannot pass through its narrow streets and alleyways."This is a different battle with enormous implications for civilians," Lise Grande told Reuters by telephone. "We have to face the possibility of a siege in the old city."She said the United Nations had considered several options for ensuring civilians' access to food, medicine and water in such a scenario.In Mosul and previous battles against Islamic State, the military has surrounded the enemy but left an escape route for fighters to reduce civilian casualties and destruction to homes and infrastructure.