US Defence Secretary Jim Mattis arrived Tuesday in Baghdad on an unannounced visit as Iraqi forces pressed an assault on Tal Afar, the Islamic State group's last major bastion in the country's north.

Mattis flew in for talks with Prime Minister Haider al-Abadi and other top officials, as well as Massud Barzani, president of the Iraqi Kurdistan Region, saying he wants to help keep the regime focused on eradicating IS jihadists.