The UN chief appointed a new special envoy to Yemen as pressure grows to return to peace talks while fighting continues in the Arab world’s poorest country.

A statement on Saturday says Ban Ki-moon has appointed Ismail Ould Cheikh Ahmed of Mauritania, who until now has led the UN’s Ebola mission.

More than a thousand people have been killed in recent weeks after Iran-backed Shiite rebels swept through the country and a Saudi-led Sunni coalition began airstrikes to drive them back.

The Western-backed president fled the country as the Houthi rebels closed in, and warnings have since grown of a humanitarian crisis as food and fuel supplies run short.

Shiite rebels have pressed an offensive in the south and a Saudi Arabia-led coalition intensified its airstrikes less than two days after it said it was scaling back the campaign.

Ahmed replaces Jamal Benomar, who had said he was stepping down. Benomar had faced sharp criticism from Saudi Arabia and other Gulf countries as his recent efforts to broker peace showed little success, though for a time Yemen had been held up as a model country for its post-Arab Spring political transition.

Benomar’s four years of efforts fell apart amid the Houthi rebel uprising and the airstrike response, which has led to fears of a kind of proxy war between Saudi Arabia and its Sunni allies and Iran, a Shiite power that has supported the Houthis.

Yemen’s UN ambassador, Khaled Alyemany, told the AP earlier this month that Benomar had not paid enough attention to the government of President Abed Rabbo Mansour Hadi, Yemen’s internationally recognized leader, and "had started to promote the Houthis, and we cannot accept that." At the time, Benomar did not comment.

Alyemany called Amhed "a very good U.N. diplomat and expert."

Ahmed was appointed by Ban in December as head of the U.N.

Mission for Ebola Emergency Response, based in Ghana. His approach in responding to the crisis was "hands-on," said Tolbert Nyenswah, head of the Ebola response in Liberia, the country that has seen more deaths than any other during the outbreak.

Nyenswah recalled a visit to Liberia’s western Grand Cape Mount County, which borders Sierra Leone and continued to record Ebola cases well after the epidemic slowed in Monrovia.