Houthi fighters opposed to Yemen's president took over the central city of Taiz in an escalation of a power struggle diplomats say risks drawing in neighbouring oil giant Saudi Arabia and its main regional rival Iran.
Residents of Taiz, on a main road from the capital Sanaa to the country's second city of Aden, said that Houthi militias took over the city's military airport without a struggle from local authorities late on Saturday.
Eyewitnesses in the central province of Ibb reported seeing dozens of tanks and military vehicles headed southward from Houthi-controlled areas toward Taiz, while activists in the city said Houthi gunmen shot into the air to disperse protests by residents demonstrating against their presence.
Conflict has been spreading across Yemen since last year when the Houthis seized the capital Sanaa and effectively removed President Abd-Rabbu Mansour Hadi, who now seeks a comeback from his base in Aden.
The advance of the Iranian-backed group has angered Sunni Gulf Arab states led by Saudi Arabia.
The Houthi spread into mostly Sunni areas in the centre and west have led to months of clashes with local tribes and al Qaeda, raising fears that the poor and heavily armed country at the base of the Arabian peninsula might descend into civil war.
The United Nations Security Council was set to discuss Yemen after Hadi, a U.S. ally, accused the Houthi militia of staging a coup and appealed to the U.N. for "urgent intervention".
Iran on Sunday called for dialogue, but suggested that Hadi should leave to spare the country further bloodshed.
"The expectation is that President ... Hadi will resign rather than repeat mistakes, to play a constructive role in preventing the break-up of Yemen and the transformation of Aden into a terrorist haven," said Iran's deputy foreign minister Hossein Amir Abdollahian, according to state news agency IRNA.
But Gulf Arab leaders and security officials on Saturday said Hadi was Yemen's legitimate ruler and they were ready to make "all efforts" to defend the country's security.
"Yemen is sliding into a dark tunnel which would have serious consequences not only on Yemen but on security and stability in the region," the officials, who included Saudi Interior Minister Prince Mohammed bin Nayef, said.
"The security of Yemen and of the GCC countries is an indivisible whole," it added.
Yemen's struggle for power intensified on Thursday, when loyalists and opponents of Hadi fought gun battles in Aden.
The fighting paused by nightfall, but suicide bombings against a Houthi mosque claimed by Islamic State militants killed nearly 140 worshippers, raising tensions and leading the Houthis to announce a military mobilisation against the militants.
On Sunday, anti-aircraft guns opened fire at an unidentified plane flying over Hadi's compound in Aden and appeared to force it away, witnesses said, in the third incident of its kind since last Thursday.
U.S. officials said Washington had evacuated its remaining personnel from Yemen, including around 100 special operations forces, because of worsening security, marking a setback in U.S. efforts against a powerful al Qaeda branch.
The Houthis are allied with former president Ali Abdullah Saleh, still influential in the military despite having given up power in 2011 after mass protests against his rule. The Yemeni army has varied loyalties, with most units being controlled by the Houthis or Saleh, while some are loyal to Hadi.