U.S. Secretary of Defense Ash Carter has arrived in Irbil, the capital of Iraq's Kurdistan region, on an unannounced visit.
Carter is due to have talks with Massud Barzani, the president of the Kurdish region, as well as meet with U.S. forces training Kurdish Peshmerga fighters.
The Peshmerga have emerged as one of the most successful groups fighting against the Islamic State (IS) extremist group in northern Iraq, winning several key battles and retaking territory from the militants.
The Peshmerga, who have received international military assistance, have repeatedly been praised by Pentagon officials for their successes against the IS.
Carter said ahead of his trip that Kurdish forces are "a powerful and successful ground force."
The Peshmerga have recently requested various new weapons from Washington, which says it is examining the requests.
Carter's visit comes one day after he met with Iraqi officials in Baghdad.
The U.S. military has estimated Iraq will eventually need three Kurdish brigades and six Iraqi Army brigades to launch an attack on the IS-held Mosul, Iraq's second-largest city.