Security sources said the militiamen, known as Huthis, had sent several thousand troops south and fought with local Sunni tribes as they approached the main southern city of Aden, where President Abedrabbo Mansour Hadi took refuge after fleeing the capital last month.
The militia yesterday seized the airport and a nearby military base in Taez, Yemen's third-largest city which is just 180 kilometres north of Aden and seen as a strategic entry point to Hadi's southern refuge.
Mounting unrest in the country -- including suicide bombings claimed by the Islamic State 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 yesterday.
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".
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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.
The Huthis moved to expand their reach in the south after taking the airport in Taez, security sources said, and had pushed toward Aden but two convoys were forced to turn back after overnight clashes with tribesmen.
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, about 80 kilometres from the city.