US President Barack Obama has tapped veteran Iraq expert Brett McGurk to coordinate the troubled US campaign against the Islamic State group, the White House says.
Announcing the departure of General John Allen, who steps down after a year marked by setbacks, policy reversals and increasing regional chaos, Obama on Friday (local time) described McGurk as "one of my most trusted advisers on Iraq."
Allen played a key role in trying to hold together the disparate 65-member coalition who have vowed to roll back Islamic State's territorial gains.
The bloc has sometimes struggled, with the White House reluctant to dramatically gear up US involvement and key members of the coalition holding divergent aims and differing degrees of commitment.
The task has only become more complex since Russia and Iran have intervened to prop up the Syrian government and deepen ties with Baghdad.
Obama offered his "profound gratitude" to Allen, who spent 38 years in the Marines, saying he had met the "challenge with tremendous ability and courage."
McGurk had most recently been Allen's deputy, focusing largely on efforts to with Sunni tribal leaders and the Iraqi government to take back Ramadi.
It is unclear if his appointment signals a rethink in policy, which critics say has floundered in recent months.
Obama recently scrapped a US$600 million mission to train Syrian opposition fighters to take on Islamic State.
The group controls swathes of northern Syria and is within striking distance of Baghdad.
On Thursday, a serviceman became the first American to be killed in action since the fight began in Iraq in June 2014.
He was in a contingent of Kurdish and US forces that stormed a prison in northern Iraq, freeing some 70 captives who were facing imminent execution, the Pentagon said.