Yemen's President, Abd-Rabbu Mansour Hadi, on Wednesday fled his palace in Aden as Iranian-backed Houthi rebels advance towards the city, the BBC reported.
Gunfire could be heard around the city center, and security forces allied to the Houthis have taken over the international airport, according to the report.
The rebels have made rapid gains since seizing a key airbase only 60 kilometers (37 miles) from Aden on Wednesday morning.
Government officials, however, deny reports that the president has fled the country, and say he remains in Aden.
The State Department said it was in touch with President Hadi earlier in the day. It said he is no longer at the compound but could not confirm any "additional details" about his location.
State television, which is controlled by the rebels, announced a reward of 20 million Yemeni riyals ($93,000) for anyone who captures the "fugitive" president.
Iran is reportedly seeking to use the Houthis to gain control over Yemen in a similar way in which it has used its proxy terror group Hezbollah to gain control in Lebanon.
Yemeni Foreign Minister Riad Yassin has called on Arab nations to stage an urgent military intervention, according to the BBC.
However, sources in neighboring Saudi Arabia told Reuters that there were no plans for military intervention in the crisis, and that the build-up of military forces on its border with Yemen was "purely defensive".
Hadi had taken refuge in Aden last month after fleeing Sanaa, where he had been under house arrest since the rebels captured the capital.
On Wednesday morning, rebel fighters secured al-Anad air base, with the support of forces loyal to ousted former President Ali Abdullah Saleh.
U.S. and European military advisers had been evacuated from the base last week after terrorists from Al-Qaeda in the Arabian Peninsula (AQAP) overran a nearby town.
Hours after the rebels captured the base, unidentified warplanes targeted the palace compound in Aden, officials and witnesses said, in the third such attack in a week.