The commander of Iraq's air force says Iraqi and coalition planes have stepped up airstrikes on Islamic State targets in the town of Tal Afar, ahead of a planned offensive there.

Lt. Gen. Anwar Hama says the planes attacked the IS group headquarters, tunnels and weapons' stores on Tuesday.

This comes as Iraqi forces are building up around the IS-held town west of Mosul, one of the last pockets of militant-held territory in Iraq after victory was declared against the militants in Mosul in July.

The United Nations says some 49,000 people have fled the Tal Afar district since April.

According to the U.N., over the course of the nine-month operation to retake Mosul, Iraq's second-largest city, nearly a million people were displaced from in and around the city.