The US State Department made the announcement on Friday after President Barack Obama underlined the importance of an international coalition that included Arab and Muslim states.

State Department deputy spokeswoman Marie Harf told reporters: "In this role General Allen will help continue to build, coordinate and sustain a global coalition across the multiple lines of efforts to degrade and ultimately destroy ISIL."

Allen served in a key US military role in the Iraq war and directed US troops as well as the International Security Assistance Force in Afghanistan from 2011 to 2013.

In late 2012 Obama nominated Allen to be supreme commander of transatlantic alliance NATO - one of the most prestigious US military posts - but he withdrew from consideration in the wake of a relationship scandal and subsequent inquiry that also led to the resignation of David Petraeus as CIA director.

Allen retired from the military in February 2013, after he oversaw the withdrawal of 34,000 US surge troops sent to war-torn Afghanistan.

The retired general will report to Secretary of State John Kerry with the goal to "work closely with the Department of Defense to match specific campaign requirements and coalition needs with potential contributors," said Harf.

Kerry is convening a meeting in Paris with a number of nations on how best to combat IS.

bk/jm (AFP, dpa)