The U.N. says former German President Horst Kohler has resigned from his role as the secretary-general's personal envoy for the disputed Western Sahara for health reasons.

U.N. spokesman Stephane Dujarric says Secretary-General Antonio Guterres spoke to Kohler on Wednesday and expressed deep regret at his resignation and "extended his best wishes to the personal envoy."Dujarric gave no details of the health reasons for Kohler's resignation.

A politician from Germany's Christian Democratic Union, Kohler served as president of Germany from 2004 to 2010.

Guterres appointed him in August 2017 to the difficult job of trying to resolve the decades-long dispute between Morocco and the Polisario Front independence movement over mineral-rich territory of Western Sahara. Dujarric said Guterres expressed "profound gratitude" to Kohler for his efforts.