NewYorktimes--  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 group, 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.

Three-quarters of the population — more than 22 million people — need food and other aid, including 11.3 million “in acute need who urgently require assistance to survive,” the United Nations humanitarian relief agency said Sunday in its latest update. It said 1.8 million Yemeni children under the age of 5 were “acutely malnourished.”

Mr. Ahmed has informed Secretary General António Guterres of the United Nations that “he does not intend to continue in his position beyond the end of his current contract ending in February 2018,” Mr. Guterres’s office said in a brief statement. It expressed thanks to the envoy “for his strong and determined support to reach a political solution to the conflict that has engulfed the country.”