UN chief Ban Ki-moon called for an immediate ceasefire in Yemen as he began the hunt for a new peace envoy to the war-torn country, where Al-Qaeda is expanding its territory.

The plea follows more than three weeks of air strikes by a Saudi-led coalition aimed at driving back Iran-backed rebels whose sweeping advance forced President Abedrabbo Mansour Hadi to flee abroad.

Yemen "is in flames," Ban told the press in Washington on Thursday. "I am calling for an immediate ceasefire in Yemen by all parties."

His remarks followed the resignation of his envoy to Yemen, Jamal Benomar, a Moroccan-born career diplomat who lost the confidence of Saudi Arabia and its allies, amid accusations that he was appeasing the rebels.

Ban said he was in the process of finding a new envoy "who can be immediately deployed" to seek a political solution.

"The Saudis have assured me that they understand there must be a political process," he said.

"I call on all Yemenis to participate, and in good faith."

Benomar had been instrumental in negotiating a deal that eased president Ali Abdullah Saleh from office in February 2012 after a year of bloody protests against his three-decade rule.

Renegade army units loyal to Saleh have allied with the Huthi rebels, providing vital support as they have advanced out of their bastion in the northern mountains near the Saudi border into mainly Sunni areas.

On Tuesday, the UN Security Council slapped an arms embargo on the Huthi rebels and demanded they give up territory they had seized.

The Yemen conflict has sent tensions soaring between Saudi Arabia and Iran -- the foremost Sunni and Shia Muslim powers in the Middle East.

Tehran is a key ally of the Huthis but vehemently denies arming them.

Yemen has sunk further into chaos since the start of the air raids by a clutch of Gulf monarchies as well as Egypt, Jordan, Morocco and Sudan.

Iraqi Prime Minister Haider al-Abadi, a Shia who has offered a rare voice of opposition to the campaign among Arab leaders, called Thursday for a political solution to quickly end the conflict.

"We are on the same boat in the region. If anybody makes a hole in that boat, we all will sink," he said, speaking at a Washington think-tank.

"In our own belief we think an end to this war of Yemen must be very soon and the only way forward is a political solution by Yemenis themselves."

Yemen is a front line in the US war on Al-Qaeda, which has exploited the growing turmoil to expand its control of areas in the southeast of the deeply tribal Arabian Peninsula country.

Yemen-based Al-Qaeda in the Arabian Peninsula (AQAP) seized the airport Thursday in the southeastern province of Hadramawt, where it now controls almost all of the provincial capital Mukalla, a port city of 200,000.

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 government's collapse forced the United States to close its embassy and withdraw US special operations forces from Al-Anad air base where they had been helping Yemen battle AQAP.

At least 20 rebels were killed, and two tanks and four armoured vehicles destroyed, in overnight air strikes on a convoy headed out of Al-Anad, officials said.

In the central city of Taez, at least 16 people were killed as pro-Hadi soldiers put up fierce resistance to a rebel attack on their camp.

Three civilians were among the dead when a stray shell hit their home, a military source and residents 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".