The retired Marine Corps four-star general was named as the special presidential envoy by Secretary of State John Kerry, who is in the Middle East trying to mobilise regional support for Washington's expanding battle against the radical group, Xinhua quoted State Department spokesperson Marie Harf as saying.
"In this role, General Allen will help continue to build, coordinate and sustain a global coalition across the multiple lines of effort to degrade and ultimately destroy ISIL," Harf told reporters at a daily press briefing, using another acronym for the group.
US: General Allen will help continue to build, coordinate the coalition
A broad coalition of partners is what Washington is seeking to support President Barack Obama's strategy for "degrading and ultimately destroying" the Islamic State, in which he vowed to expand the ongoing airstrikes in Iraq and take action against the group in Syria.
Harf said Brett McGurk, deputy assistant secretary of state for Near Eastern affairs, will serve as Allen's deputy.
Allen was commander of the International Security Assistance Force as well as US forces in Afghanistan from July 2011 to February 2013. He was deployed to Iraq years before.
The general was nominated to head NATO's Supreme Allied Commander, Europe in early 2013.