The whereabouts of Yemen's beleaguered president, Abed Rabbo Mansour Hadi, are unknown after he fled his presidential compound in the southern city of Aden. Forces loyal to the president have vowed to fight.
On Wednesday, forces from the rebel Houthis - a Shiite tribe from the country's mountainous north - continued to advance southward, captured a key air base and the country's defense minister, and began closing in on the southern city of Aden, where Yemen's government has been based since it was dislodged from the capital, Sanaa, last year.
Local residents reported that sounds of gunfire and explosions were heard at a Yemeni army base in Aden on Wednesday and that Houthi fighters were within about 20 kilometers (12 miles) of the city's northern entrance.
"The war is imminent and there is no escape from it," 21-year-old Mohammed Ahmed told Reuters as he stood outside a security compound in Aden's Khor Maksar district, where hundreds of young men have been signing up to fight the advancing Houthi fighters.
Forces loyal to the president have been firing into the air at Aden's Jabal al-Hadeed barracks to prevent residents from intruding into the military base and arming themselves.
Before the clashes in the city center, unidentified warplanes fired missiles at the Aden neighborhood where Hadi's compound is located, and anti-aircraft batteries opened fire on the planes.
In the meantime, Houthi militia forces advanced to the south to Dar Saad, a village a half-hour's drive from the city center of Aden.
This comes as the US and Britain evacuated its special forces from an air base 60 kilometers (37 miles) from the southern city of Aden. Warplanes attacking the city have forced the civil airport to close.
The fighting now threatens to plunge Yemen into a full-scale civil war, with soldiers loyal to Hadi facing off against Houthi rebels.
Hadi came to power in 2011 and is backed by most Western powers and neighboring Gulf Arab countries.
Allegations that Houthis receive support from Iran have raised concerns that the Arab world's poorest nation could turn into a proxy battleground between Tehran and Riyadh.
Hadi government asks for help
Meanwhile, Hadi's chief diplomat is in Egypt ahead of a two-day Arab League summit to plead for support. "We will ask the upcoming summit for urgent intervention," acting Foreign Minister Riyadh Yassin told reporters in the resort city of Sharm el-Sheikh, where the summit will begin Saturday. "Military intervention," he stressed.
The chaos in Yemen has interrupted US operations in the country, including the CIA's drone program, which had been using its air base and intelligence on the ground to strike and kill suspected Islamist militants.
Saudi Arabia, a main backer of Hadi and an arch-foe of Iran, has already started massing troops along its border.
Saudi Arabia's foreign minister, Saud al-Faisal, warned that "if the Houthi coup does not end peacefully, we will take the necessary measures for this crisis to protect the region."
Yemen's state television broadcaster, controlled by Houthis, have put out a $100,000 (91,000 euros) for Hadi's capture as rebel Houthis set up positions, poised to overrun the presidential compound.
jar/kms (AP, AFP, Reuters)