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A file picture taken on September 19, 2014 shows UN peace envoy to Yemen Jamal Benomar preparing to ride in a plane in Sanaa. Benomar, has resigned, a UN official said on April 16, 2015 after losing support from Gulf countries for his mission in the conflict-riven nation. The Moroccan diplomat had been UN Secretary General Ban Ki-moon's special envoy for Yemen since 2012. Image Credit: AFP

United Nations: The UN’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 militia uprising and Saudi-led air strikes.

A UN statement late Wednesday said Jamal Bin Omar “has expressed an interest in moving on to another assignment” and that his successor will be named “in due course.” Bin Omar’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 UN staff pulled out.

Bin Omar, 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.

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

The UN said in its statement 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 Al Houthis. More than 700 people have been killed since the air strikes began.

UN 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 Bin Omar.

The pressure on Bin Omar, 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 UN was strongly hinting that Bin Omar 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 Bin Omar, yes,” the ambassador, Abdallah Al-Mouallimi, told reporters Friday.

Bin Omar, 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, Bin Omar earned praised as Yemen seemed like a model for such a transition, but as the new UN statement pointed out, “unfortunately, this process was interrupted with the dramatic escalation of violence.”

The UN 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 UN-guided transition and now has aligned himself with the Houthis.

The Gulf Cooperation Council - which includes Yemen’s neighbors 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 UN-led negotiations and full implementation of the plan, which includes drafting a constitution and elections.