It is expected to be the biggest battle since the US-led invasion in 2003.

Troops from Iraq's elite Counter-Terrorism Forces and US military personnel were seen taking up positions to the west of the Kurdish city of Erbil.

Iraqi officials say a massive ground assault could begin this month.

The operation's being backed by US air power, Kurdish security forces and Shi'ite and Sunni irregular units.

With a pre-war population of around two million, Mosul is at least five times bigger than any other city Islamic State has controlled.

A successful operation would effectively destroy the Iraqi half of the caliphate declared by the militants when they swept through northern Iraq in 2014.

But it's not without risks.

The group has a history of using civilians as human shields to defend territory.

The United Nations is warning the assault could uproot a million people, unleashing the world's biggest humanitarian crisis.