The U.N.'s special envoy to Yemen has stepped down after four years of efforts at a peaceful political transition in the Arab world's poorest country fell apart amid a Shiite rebel uprising and Saudi-led airstrikes.

A U.N. statement said Jamal Benomar "has expressed an interest in moving on to another assignment" and that his successor will be named "in due course."

U.N. spokesman Stephane Dujarric would not comment on published reports that the head of the U.N.'s Ebola emergency response mission in West Africa, Ismael Ould Cheikh Ahmed of Mauritania, would replace Benomar. He said the U.N. secretary-general would consult the parties in Yemen and countries in the region before selecting a replacement, adding that it must be "someone who can talk to all parties."

Benomar's departure creates a diplomatic vacuum in Yemen, where he had been the key international figure working to bring the feuding parties together, even after diplomats fled embassies and the U.N. staff pulled out.

Benomar, who previously served as an envoy in Iraq and Afghanistan, had come under criticism from some in the Middle East, particularly Saudi Arabia, as his recent efforts to broker peace showed little success. Dujarric said Thursday he could not say whether Benomar would get another U.N. job.

Yemen is now under weeks of airstrikes by a Saudi-led coalition in an attempt to push back Shiite Houthi rebels who swept south and caused the Western-backed president to flee.

The U.N. said in its statement late Wednesday that it will "spare no efforts to re-launch the peace process," but the challenge has grown as the fighting in Yemen has become a kind of proxy war between Saudi Arabia and its Sunni allies and Iran, a Shiite power that has supported the Houthis. More than 700 people have been killed since the airstrikes began.

U.N. diplomats, speaking on condition of anonymity because talks were private, said that ministers from the Sunni-led Gulf Cooperation Council met Secretary-General Ban Ki-moon during a trip to Kuwait in late March and told him of their unhappiness with Benomar.

The pressure on Benomar, a Moroccan-born diplomat who holds British citizenship, had grown as the Houthis advanced in recent weeks. By late last week, Saudi Arabia's ambassador to the U.N. was strongly hinting that Benomar was on the way out.

"We continue to support the mission of the special adviser to the secretary-general. ... Whoever the secretary-general designates as his special adviser, for the time being Jamal Benomar, yes," the ambassador, Abdallah Al-Mouallimi, told reporters Friday.

Benomar, who was held as a political prisoner while a student in Morocco, had been tasked in 2011 with guiding a peaceful transition for Yemen from the chaos of the Arab Spring. For a while, Benomar earned praised as Yemen seemed like a model for such a transition, but as the new U.N. statement pointed out, "unfortunately, this process was interrupted with the dramatic escalation of violence."

The U.N. Security Council this week imposed an arms embargo on Houthi leaders and again demanded that they withdraw and stop the violence. The council also imposed an arms embargo on former president Ali Abdullah Saleh, who had stepped down in early 2012 as part of the U.N.-guided transition and now has aligned himself with the Houthis.

The Gulf Cooperation Council -- which includes Yemen's neighbours Saudi Arabia and Oman as well as Bahrain, Kuwait, Qatar and the United Arab Emirates -- put together the plan for a political transition in Yemen that was only partially carried out. The Security Council this week called for a return to U.N.-led negotiations and full implementation of the plan, which includes drafting a constitution and elections.