Al-Qaeda militants seized a provincial airport Thursday in war-wracked Yemen, where the jihadist network's deadly local franchise has exploited growing instability to expand its foothold.
Yemen-based Al-Qaeda in the Arabian Peninsula has sought to tighten its grip on the largely lawless southeast since an armed rebellion sweeping the country added to a breakdown in security.
The extremists now control almost all of Mukalla, a city of 200,000 and the capital of the southeast province of Hadramawt, including its airport.
"The military unit in charge of security at the facility withdrew without resisting," an airport official said.
For years, Yemen has been a key US ally in the fight against Al-Qaeda, allowing Washington to carry out drone attacks on its territory.
But the deeply tribal country on the southern tip of the Arabian Peninsula sank further into chaos after a Saudi-led regional coalition launched air strikes aimed at driving back Iran-backed Shiite rebels on March 26.
Elsewhere in Hadramawt, tribal forces took control of a major oil terminal in Al-Shihr after soldiers protecting it withdrew, a military source said.
The air raids have so far failed to beat back the rebels, known as Huthis, whose sweeping advance southwards from their northern stronghold since last year prompted President Abedrabbo Mansour Hadi to flee to Riyadh last month.
The UN said Wednesday that its envoy to Yemen, Jamal Benomar, had resigned after failing to avert large-scale violence, dealing a blow to hopes of a diplomatic solution.
Benomar had retained the support of UN chief Ban Ki-moon, who has repeatedly called for a return to the negotiating table, but he lost the confidence of Riyadh and its allies.
Last month, a Gulf diplomat accused him of appeasing the rebels and their allies as they overran Saudi Arabia's impoverished but strategically important neighbour.
The Moroccan diplomat had been instrumental in negotiating a peace deal that eased president Ali Abdullah Saleh from office in February 2012 after a year of bloody protests against his three-decade rule.
- Envoy 'failed his mission' -
Among the candidates to replace him is Mauritanian Ismail Ould Cheikh Ahmed, who currently heads the UN Ebola mission in Accra, a UN official said.
Signup to our newsletter and follow us on Facebook and Twitter!
London-based analyst Abdulwahab Badrakhan said Benomar resigned after he "failed in his mission," especially after the Huthis overran Sanaa in September.
He had been unable to convey "the reality and goals of the Huthis to the Security Council and the international community," Badrakhan told AFP.
This led to the "deteriorating security and political situation" that finally "called for a regional intervention and plunged the country into civil war," he said.
The resignation came hot on the heels of the adoption by the UN Security Council of a resolution that the Saudi-led coalition saw as support for its bombing campaign.
The resolution -- the first formal action taken by the Security Council since the air strikes started -- demands that the rebels withdraw from Sanaa and all other areas they have seized.
It also slapped an arms embargo on the rebels and army units still loyal to ex-president Saleh who have allied with them, providing crucial support as they have advanced out of their bastion in the northern mountains into mainly Sunni areas.
Hadi's newly appointed vice president, Khaled Bahah, urged those army units to drop their support for the Huthis.
"I call on all troops and security force personnel to accept the command of the legitimate government and protect the country," he said in Riyadh, where he is also exiled.
- Plea for aid -
Troops and militia loyal to Hadi have been battling the rebels in Aden and other southern provinces.
Overnight, coalition aircraft carried out fresh air strikes on rebel positions in Aden, killing at least eight rebels, a military source said.
In southwest Yemen, rebel soldiers loyal to Saleh traded tank and artillery fire with a pro-Hadi brigade in the city of Taez, military sources and witnesses said.
The World Health Organization says at least 736 people died in the conflict up to April 12 and more than 2,700 had been wounded.
Aid has been slow to trickle into the country, but the WHO said it had delivered more than 17 tonnes of medical aid by plane to Sanaa on Thursday.
It said hospitals "are experiencing a rapid depletion of their stocks as they witness a surge in the number of injured patients in acute need of life-saving medical and surgical interventions".