Mr. Carter is in Iraq to meet with Iraqi leaders and military commanders
The US defence secretary has arrived on an unannounced visit to Iraq to assess the progress of the offensive against so-called Islamic State (IS) in Mosul.
Ash Carter comes fresh from meetings with Turkish leaders in Ankara aimed at allowing Turkey to play a part in the Mosul operation despite Iraqi concerns.
He has overseen a steady increase in US troop numbers in Iraq this year.
Iraqi forces have entered the town of Qaraqosh, 32km (20 miles) south of Mosul, commanders say. Qaraqosh, Iraq’s largest Christian town before the war, is said to be largely empty but IS has mined the approaches to Mosul.
The militants have been attacking with suicide bombers elsewhere, driving vehicles laden with explosives at high speed towards government lines.
Mr Carter, on his third trip to the country in a year, is due to meet Iraqi leaders and military commanders.
More than 4,800 US soldiers are in Iraq and at least 100 US special operations personnel are operating with Iraqi units.
The offensive against Mosul, which began on Monday, is a two-pronged operation, with Iraqi government forces attacking from the south and Kurdish fighters advancing from the east.
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