Houthi militia forces in Yemen backed by allied army units seized an air base on Wednesday and appeared close to capturing the southern port of Aden from defenders loyal to President Abd-Rabbu Mansour Hadi, residents said.
The United States said that Hadi, who has been holed up in Aden since fleeing the Houthi-controlled capital Sanaa last month, was no longer at his residence. It offered no other details on his movements.
After taking al-Anad air base, the Houthis and their military allies, supported by heavy armour, advanced to within 20 km (12 miles) of Aden.
Soldiers at Aden's Jabal al-Hadeed barracks fired into the air to prevent residents from entereing the base and arming themselves, witnesses said, suggesting that Hadi's control over the city was fraying.
Houthi fighters and allied military units had advanced to Dar Saad, a village a half-hour's drive from central Aden, residents there said.
Earlier, unidentified warplanes fired missiles at the Aden neighbourhood where Hadi's compound is located, residents said. Anti-aircraft batteries opened fire on the planes.
The city's airport was closed and all flights were cancelled for security reasons, guards at the facility told Reuters.
Yemen's slide towards civil war has made the country a crucial front in mostly Sunni Saudi Arabia's rivalry with Shi'ite Iran, which Riyadh accuses of stirring up sectarian strife through its support for the Houthis.
Sunni Arab monarchies around Yemen have condemned the Shi'ite Houthi takeover as a coup and have mooted a military intervention in favour of Hadi in recent days.
U.S. officials say Saudi Arabia is moving heavy military equipment including artillery to areas near its border with Yemen, raising the risk that the Middle East's top oil power will be drawn into the worsening Yemeni conflict.
Saudi sources said the build-up, which also included tanks, was purely defensive.
While the battle for Aden is publicly being waged by the Houthi movement, many there believe that the real instigator of the campaign is former president Ali Abdullah Saleh, a fierce critic of Hadi.
It was Saleh who was the author of Aden's previous humiliation in 1994, when as president he crushed a southern secessionist uprising in a short war.
Unlike other regional leaders deposed in the Arab Spring, Saleh was allowed to remain in the country.
Army loyalists close to Saleh on Wednesday warned against foreign interference, saying on his party website that Yemen would confront such a move "with all its strength".
Diplomats say they suspect the Houthis want to take Aden before an Arab summit this weekend, to preempt an expected attempt by Hadi ally Saudi Arabia to rally Arab support at the gathering for military intervention in Yemen.
Yemeni officials denied reports that Hadi had fled Aden.
But in Washington, a U.S. State Department spokeswoman said Hadi had left his residence.
"We were in touch with him earlier today," spokeswoman Jen Psaki told a briefing. "He is no longer at his residence. I'm not in position to confirm any additional details from here about his location."
Hadi left the residence voluntarily, she said.
The Arab League will on Thursday discuss a proposal by Yemen's foreign minister, who called on Arab states to intervene militarily to halt the Houthi advance, the regional body's deputy secretary general said.
The Houthi advance was taking its toll. The bodies of fighters from both sides lay on the streets of the outskirts of Houta, capital of Lahej province north of Aden, residents said.
In Houta, storefronts were shuttered and residents reported hearing bursts of machine gun fire and saw the bodies of fighters from both sides lying in the streets.
Witnesses said Houthi fighters and allied soldiers largely bypassed the city centre and travelled by dirt roads to the southern suburbs facing Aden.
In Aden, heavy traffic clogged Aden as parents brought schoolchildren home and public sector employees obeyed orders to leave work. Witnesses said pro-Hadi militiamen and tribal gunmen were out in force throughout the city.
"The war is imminent and there is no escape from it," said 21-year-old Mohammed Ahmed, standing outside a security compound in Aden's Khor Maksar district, where hundreds of young men have been signing up to fight the advancing Shi'ite fighters.
"And we are ready for it.
The northern Houthi militia alongside army units loyal to Saleh have driven back an array of tribal fighters, army units and southern separatist militiamen loyal to Hadi.
Houthi militants took control of Sanaa in September and seized the central city of Taiz at the weekend as they moved closer to Aden.
Houthi leaders have said their advance is a revolution against Hadi and his corrupt government, and Iran has blessed their rise as part of an "Islamic awakening" in the region.
While Hadi has vowed to check the Houthi push south and called for Arab military support, his reversals have multiplied since heavy fighting first broke out in south Yemen on Thursday and the Houthis began making rapid advances southward.