Parts of Yemen's third largest city Taez have been seized by Shia Huthi rebels.
Parts of Yemen's third largest city Taez have been seized by Shia rebels, reports say.
The city's airport was among the areas seized by Huthi rebels as Yemen's deteriorating security prompted Washington to evacuate personnel and the UN Security Council to call an emergency session.
The Security Council was to meet later on Sunday after President Abedrabbo Mansour Hadi called for "urgent intervention" amid mounting unrest, including suicide bombings claimed by the Islamic State group that killed 142 people in the capital Sanaa on Friday.
Impoverished but strategic Yemen has descended into chaos in recent months, with the Shi'ite militia, known as Huthis, seizing control of Sanaa and forcing Hadi to flee to the main southern city of Aden.
The Arabian Peninsula country is increasingly divided between a north controlled by the Huthis, who are allegedly backed by Iran, and a south dominated by Hadi's allies.
On Sunday the Huthis and their allies seized Taez's airport, which is just 180km north of Aden on the road to Sanaa and seen as a strategic entry point to Hadi's refuge.
Security sources told AFP some 300 men, including Huthi fighters dressed in military uniforms and allied forces, had deployed at the airport and reinforcements were arriving from Sanaa by air and land.
The forces allied with the Huthis included members of the former central security force, a unit seen as loyal to ex-president Ali Abdullah Saleh.
Saleh was forced from power in early 2012 after a year-long popular uprising and has been accused of working with the Huthis to restore his influence.
Security sources said Huthi militiamen were also patrolling parts of Taez and had set up checkpoints in Raheda, some 80km south of the city on the road to Aden.
A military source said troops loyal to Hadi and southern paramilitary forces had meanwhile deployed in Lahj province, north of Aden, in anticipation of a possible advance by the Huthis.
Hadi, backed by Western and Gulf states as Yemen's legitimate ruler, has been struggling to reassert his authority since escaping house arrest in Sanaa last month and fleeing to Aden.
In a letter to the Security Council, he said the Huthis and their allies "not only threaten peace in Yemen but regional and international peace and security".
He called for "urgent intervention by all available means to stop this aggression that is aimed at undermining the legitimate authority, the fragmentation of Yemen and its peace and stability."
The country is increasingly divided along sectarian lines, with the Shi'ite militia facing resistance from Sunni tribesmen and Al-Qaeda in the Arabian Peninsula (AQAP), the powerful branch of the global jihadist network.
Yemen has long been a key US ally in the fight against Islamic extremism, allowing Washington to carry out drone strikes on AQAP on its territory.