US Defence Secretary Ashton Carter arrived in Baghdad on Thursday for talks with top political and military officials on Iraq's war against the Islamic State jihadist group.

Carter, on his first visit to Iraq since taking the job in February, is expected to meet Prime Minister Haider al-Abadi, as well as Sunni tribal leaders from Anbar province, where much of the current fighting is taking place.

Top foreign officials' visits to Iraq are generally not announced in advance for security reasons.

Carter is expected to meet some of the 3,500 US troops deployed in Iraq to advise and train local forces battling IS.

Washington leads an international coalition that has carried out thousands of air strikes against IS targets in Iraq and Syria over the past year.

Baghdad's other main partner in the fight against IS is Tehran, which also has advisers on the ground and directly supports the powerful Shiite militias now playing a lead role in the military effort.

Iran is not part of the coalition, and while this month's historic nuclear deal opened a new chapter in relations between Tehran and Washington, their coexistence on Iraq's battlefield remains uneasy.