Yemen's Shiite militia sent reinforcements south and clashed with local fighters on Monday after the United Nations warned the country is on the brink of a protracted civil war.
Security sources said the militiamen, known as Huthis, had sent thousands of troops and clashed with Sunni tribes, with their sights set on the main southern city of Aden, where President Abedrabbo Mansour Hadi fled to from the capital last month.
The militia on Sunday seized the airport and a nearby military base in Taez, 180 kilometres (110 miles) north of Aden and seen as a strategic entry point to Hadi's southern refuge.
Yemen, a long-time US ally which borders Saudi Arabia, is increasingly divided between a north controlled by the Huthis, who are allegedly backed by Iran, and a south dominated by Hadi supporters.
Mounting unrest -- including suicide bombings claimed by the Islamic State (IS) jihadist group that killed 142 people in the Huthi-controlled capital Sanaa on Friday -- have raised international concerns and prompted an emergency session of the UN Security Council on Sunday.
Its 15 members voiced their unanimous support for Hadi, with UN envoy Jamal Benomar warning that without immediate action the country will slide into "further violence and dislocation".
"(Recent events) seem to be leading Yemen to the edge of a civil war," Benomar told the meeting by video link from Qatar, warning of a protracted crisis like "a Libya-Syria combined scenario".
The Huthis seized Sanaa in September and have been expanding their territory, clashing with Hadi loyalists, local tribes and Sunni Islamist groups including Al-Qaeda.
Since taking over Taez airport, the militia has pushed toward Aden, security sources said, although two convoys were repelled in overnight clashes with tribesmen.
- 'Puppet in hands of evil' -
It was not immediately clear how many Huthis or tribesmen may have been killed or wounded in the clashes near Al-Abd, about 40 kilometres from Taez, and Al-Maqatara, 80 kilometres from the city.
The Huthis have also deployed some 5,000 militiamen and more than 80 tanks to an area of neighbouring Ibb province, 30 kilometres northeast of Taez, local and military sources said.
The reinforcements have converted schools into barracks, the sources said.
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Troops and southern paramilitary forces loyal to Hadi have also reportedly deployed in Lahj province, north of Aden, to ward off any Huthi advance.
Huthi leader Abdulmalik al-Huthi in a televised speech Sunday called for his supporters to mobilise for an offensive in the south, condemning Hadi as "a puppet in the hands of forces of evil, led by the United States".
Western countries and Sunni Arab Gulf monarchies have backed Hadi as the country's legitimate ruler.
On Monday, Saudi Foreign Minister Prince Saud al-Faisal reiterated calls for Huthis to "obey an international consensus on rejecting the coup," in reference to their seizure of power in Sanaa.
- 'Journalist arrests' -
Yemen has been a key US ally in the fight against Al-Qaeda by allowing Washington to carry out drone strikes against jihadists on its territory.
Washington announced Saturday it was evacuating its remaining personnel from Yemen, underlining fears in the West of growing insecurity.
The forces allied with the Huthis include members of the former central security force, a unit seen as loyal to ousted 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.
Human Rights Watch on Monday accused the Huthis of an increasing number of attacks on journalists, including arrests and alleged torture.
"There has been an increase in arbitrary arrests and violence against journalists and other media workers" by Huthis, HRW said.
The Huthis reportedly killed one protester in Taez on Sunday when they fired on thousands demanding they withdraw.
Activists said several thousand Taez residents protested Monday outside a camp of pro-Saleh forces. Witnesses said four demonstrators were wounded.
Elsewhere, IS claimed responsibility for an attack Friday in Lahj that killed 29 police, in a sign of its growing activity in Yemen.
The attack came on the same day that two Shiite Huthi mosques were bombed in Sanaa. On Monday, IS posted pictures of what it said were the five Yemeni suicide bombers on the Internet.