Yemen's Houthi fighters have seized parts of the southern city of Taiz and its airport and are pushing to seize more territory across the country, Al Jazeera has learnt.
The Houthis seized the airport in Taiz, the country's third-largest city on Sunday after clashes with forces loyal to President Abd-Rabbu Mansour Hadi.
Tens of tanks and armoured personnel carriers carrying Houthi fighters had crossed into al-Dhalie and Aden governorates, Al Jazeera's correspondents had said.
The airport was seized amid demonstrations against both the Houthis and ex-president Ali Abdullah Saleh, who is accused of backing the Houthis.
Ahmed al-Wafi, a Yemeni political activist told Al Jazeera that the Houthis had taken full control of Taiz military airbase, which lies around 180km north of Aden, and had deployed fighters to man checkpoints at the city's entry points and streets.
Taiz is seen as a strategic entry point to Hadi's refuge.
The forces who launched the assault, include Houthi fighters allied with members of the former central security force, a unit seen as loyal to Saleh.
A military source told the AFP news agency that 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.
Meanwhile, Yemeni anti-aircraft guns on Sunday opened fire at an unidentified plane flying over Hadi's compound in Aden and appeared to force it away, witnesses cited by the Reuters news agency said.
It was the third incident of its kind in the past four days, in which unidentified aircraft have flown over the compound, where Hadi is based, on one occasion dropping bombs without causing any casualties.
'Drums of war'
A spokesperson for the Yemen embassy in Washington said the country was teetering on the brink of full-scale war.
"I hate to say this, but I'm hearing the loud and clear beating of the drums of war in Yemen," Mohammed al-Basha wrote on Twitter.
On Saturday, Hadi gave his first televised address since fleeing Sanaa, striking a defiant tone. He described the rebels' rule as "a coup against constitutional legitimacy".
He also pledged to raise the Yemeni flag over the Maran mountains, a stronghold for the Houthis, members of the Shia Zaydi sect that represents nearly 30 percent of Yemen's population.
Hadi also said regional Shia power Iran supported the Houthis, something critics also allege and the rebels deny.
Sunni Gulf countries have lined up to support Hadi and have moved their embassies to Aden to back him against the Shia rebels.
Meanwhile, the United States said it had evacuated all its staff from Yemen as Hadi appealed for "urgent intervention" by the UN Security Council.
"Due to the deteriorating security situation in Yemen, the US government has temporarily relocated its remaining personnel out of Yemen," State Department spokesman Jeff Rathke said in a statement.
Hadi, backed by Western and Gulf states as Yemen's legitimate ruler, has been struggling to reassert his authority since escaping house arrest and fleeing to Aden last month.
Source: Al Jazeera and agencies