UPDATE 1500 GMT: Residents of Aden, the port city in southern Yemen, say Houthi militia are within 20 km (12 miles) of the northern entrance.
The Shia group has reportedly advanced to Dar Saad, a village a half-hour’s drive from Aden.
Local sources also reported sounds of gunfire and explosions at an army base in the centre of the city.
Soldiers at the Jabal al-Hadeed barracks fired into the air to prevent residents from infiltrating and arming themselves, witnesses said.
Yemen’s Houthi movement, already in control of the capital Sana’a, have advanced further in the south with the capture of the country’s largest airbase.
The Houthis, a Shia Muslim group, took the al-Anad airbase near Aden from forces loyal to President Abdrabuh Mansour Hadi. The US had evacuated its forces — estimated at about 100 troops and special forces units — in anticipation of the assault.
Claims are circulating that Hadi has fled his residence in Aden, although these have been denied by his spokesperson.
The President left Sana’a last month, escaping house arrest, and set up an alternative Government in the south after the Houthis took control of the capital.
The Houthis also reportedly took al-Hawta, the capital of Lahj Province, this morning.
The UN has been trying to convene talks to resolve the situation, announcing discussions in Qatar earlier this week. That process appears to have been overtaken by the Houthi advance.
On Tuesday, Hadi asked the UN Security Council to authorise military intervention by “willing countries” to “protect Yemen and to deter the Houthi aggression expected to occur at any hour from now”. He also requested assistance from the six-nation Gulf Co-operation Council and the Arab League.
US officials say that Saudi Arabia is moving heavy military equipment, including artillery, near its border with Yemen.
(Featured Map: American Enterprise Institute)