The United Nations special envoy for Yemen, whose attempts at diplomacy to resolve the devastating war in that country have repeatedly failed, is resigning as of next month, the organization announced Monday.

The envoy, Ismail Ould Cheikh Ahmed, a veteran diplomat from Mauritania, is the second United Nations mediator in less than three years to vacate the Yemen post in frustration.

His predecessor, Jamal Benomar, a Moroccan diplomat, quit in April 2015 after four years as the special Yemen envoy. That was a month after a Saudi Arabia-led coalition began intensively bombing Yemen’s Houthi rebels, who had dislodged the Saudi-backed government and seized much of the country, including the capital, Sana.

Since then the war has ravaged Yemen, the Middle East’s most destitute country, and created what the United Nations has called the world’s biggest man-made humanitarian disaster.