US Secretary of Defence Ashton Carter made an unannounced visit to Baghdad on Monday for talks with Iraqi officials on the war against the Islamic State jihadist group.

The US is leading an international coalition that is carrying out strikes against IS and also providing training and other assistance to forces fighting the jihadists in both Iraq and neighbouring Syria.

Carter will meet officials including Prime Minister Haider al-Abadi and Defence Minister Khaled al-Obeidi to discuss increasing American military assistance to Iraq ahead of the key battle for second city Mosul.

"The fight for Mosul is critical... and it's a very large urban scenario," a senior defence official said.

"We are going to need to be more aggressive," the official said.

IS overran large areas north and west of Baghdad in 2014, but Iraqi forces have since regained significant ground from the jihadists.

Iraqi forces have begun preparatory operations in Nineveh province, of which Mosul is the capital, but the actual assault to take the city is not expected to take place for months, and possibly not until next year.

The US has deployed some 3,900 troops to Iraq, most of them in advisory and support roles.

But Washington has also sent special forces to carry out raids against IS, and US Marines have deployed to provide artillery support to Iraqi troops.