Yemen's embattled president has fled his Aden home for an undisclosed location as Shiite rebels mass near his last refuge.
The news came just hours after the rebels' own television station said they had seized an air base where US troops and Europeans advised the country in its fight against al Qaida.
That air base is only 35 miles away from Aden, the port city where President Abed Rabbo Mansour Hadi had established a temporary capital.
Witnesses said they saw a convoy of presidential vehicles leaving Mr Hadi's palace, located at the top of a hill in Aden overlooking the Arabian Sea.
The advance of the Shiite rebels, known as Houthis, threatens to plunge the Arab world's poorest country into a civil war that could draw in its Gulf neighbours. Already, Mr Hadi has asked the UN to authorise a foreign military intervention in the country.
Military officials said militias and military units loyal to Mr Hadi had "fragmented," speeding the rebel advance.
Mohammed Abdel-Salam, a spokesman for the Houthis, said their forces were not aiming to "occupy" the south. He said the rebels were fighting Mr Hadi's allied forces on five different fronts.
Earlier, Houthis and allied fighters were said to have secured the al-Annad air base, the country's largest. It claimed the base had been looted by both al Qaida fighters and troops loyal to Mr Hadi.
The reported Houthi takeover of the base took place after hours-long clashes between rival forces around the base. The US recently evacuated some 100 soldiers, including Special Forces commandos, from the base after al Qaida briefly seized a nearby city. Britain also evacuated soldiers.
The base was crucial in the US drone campaign against al Qaida in the Arabian Peninsula, which America considers to be the most dangerous branch of the terror group.
American and European military advisers there also offered logistical in its fight against the al Qaida group, which holds territory in eastern Yemen and has claimed it directed the recent attack against the French satirical magazine Charlie Hebdo in Paris.
US operations against the militants have been scaled back dramatically amid the chaos in Yemen. US officials have said CIA drone strikes will continue in the country, though there will be fewer of them. The agency's ability to collect intelligence on the ground in Yemen, while not completely gone, is also much diminished.
The takeover of the base is part of a wider offensive led by Houthis, backed by loyalists of deposed president Ali Abdullah Saleh within Yemen's armed forces.
The Houthis, in the aftermath of suicide bombings in Sanaa last week that killed at least 137 people, ordered a general mobilisation of its forces. The group's leader, Abdel-Malik al-Houthi, vowed to send his forces to the south under the context of fighting al Qaida and militant groups.
The Houthis seized the capital, Sanaa, in September and have been advancing south alongside forces loyal to Saleh.
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