CAIRO — Yemeni fighters and army units allied with the Houthi movement closed in on the last redoubt of President Abdu Rabbu Mansour Hadi of Yemen on Wednesday amid news reports that he had fled by boat across the Gulf of Aden, possibly to the tiny African nation of Djibouti.
The unconfirmed reports of Mr. Hadi’s departure were denied by his foreign minister. But they came at a moment when his exit was increasingly expected amid a cascade of events hurtling Yemen closer to civil war.
The region’s most impoverished country, Yemen has been a central theater of the American fight against Al Qaeda. Along with Syria, Iraq and Libya, Yemen is also now at least the fourth state to veer toward collapse in the aftermath of the Arab Spring revolts.
By morning, Houthi forces had seized Al Anad air base, which until recently had been used by American counterterrorism forces, about 35 miles from Mr. Hadi’s base in the southern port city of Aden.
A television network under Houthi control said they had found the base empty and looted, and had captured two senior officers loyal to Mr. Hadi, including his defense minister.
A few hours later, Yemeni Air Force planes under Houthi control struck targets near the president’s Aden home and his supporters returned fire with antiaircraft guns. The state television network, also controlled by Houthis, announced a $100,000 bounty for Mr. Hadi’s arrest as rumors about his whereabouts swirled. By nightfall, there were reports that Houthi forces were fighting around the Aden airport, on the outskirts of the city.
A security official of Mr. Hadi’s government confirmed the loss of Al Anad. Mr. Hadi’s foreign minister, meanwhile, reiterated his calls for intervention by Saudi Arabia, Egypt and other Arab states to stop the Houthis, stoking fears that their advance could trigger a widening regional conflict.
The country is now sliding toward a civil war as dangerous as any in the region, with elements of a sectarian feud, a regional proxy conflict, the attempted return of an ousted authoritarian and the expansion of anti-Western extremist groups like Al Qaeda and the Islamic State eager to capitalize on the chaos.
The Houthis, a minority religious group from northern Yemen, practice a variant of Shiite Islam and receive support from Iran.
But they are also collaborating with Yemeni security forces still loyal to former President Ali Abdullah Saleh, the longtime strongman who was pushed from power amid the Arab Spring uprising but now appears to be staging a comeback in alliance with the Houthis.
With Mr. Saleh’s help, the Houthis now control most of the Yemeni military, including its air force. That has given them a decisive advantage over Mr. Hadi’s forces, as their seizure of the Al Anad base on Wednesday made clear. But their ability to control and govern their expanding territory was far from clear, analysts said. Also unclear was the potential pushback from Saudi Arabia and other regional powers.
“I would not be surprised if we see Aden falling to the Houthis tomorrow,” said Ibrahim Sharqieh, deputy director of the Brookings Doha Center. “But that will just set the stage for a prolonged conflict or civil war, because the Houthis have not been able to maintain order even in the areas they have controlled since last year.”
Mr. Hadi, the president, was installed as a replacement for Mr. Saleh in a transition brokered by Persian Gulf monarchies, and he has the backing of both Saudi Arabia and Washington. But he fled to Aden from the capital, Sana, after the Houthis captured it months ago. He now appears to have retained the support and protection of only a small number of military units and some tribal groups based in the predominantly Sunni Muslim south.
As Mr. Hadi’s opponents close in, his supporters have escalated calls for Saudi Arabia, the other Persian Gulf states and Egypt to intervene to hold back the Houthis, portraying them as an arm of Iran.
At a news conference this week, Prince Saud al-Faisal, the Saudi foreign minister, hinted at a possible intervention, saying Saudi Arabia “will take the necessary measures for this crisis to protect the region.”
Saudi Arabia has recently begun mobilizing its military forces near the Yemeni border, although possibly for purely defensive reasons, Reuters reported on Wednesday, citing American officials.
At the Arab League, officials said Wednesday that Yemen would top the agenda at a meeting this week of the Arab foreign ministers in Sharm el Sheikh, Egypt.
But a spokesman for the Egyptian Foreign Ministry quickly dispelled rumors that Cairo had already agreed to intervention. “We have no idea at all about that,” said the spokesman, Ambassador Badr Abdellatty.
Against the backdrop of the escalating conflict, Sunni Muslim extremists pledging allegiance to both Al Qaeda and the Islamic State have been escalating their attacks, including sectarian assaults on the Houthis. The Islamic State, also known as ISIS and ISIL, claimed responsibility for two bombings of Shiite mosques in Sana on Friday that killed more than 135 people. Al Qaeda’s affiliate, Al Qaeda in the Arabian Peninsula, already effectively controls pockets of southern Yemen beyond the reach of either Mr. Hadi or the Houthis.
The United States evacuated its military personnel from Al Anad several days ago, as fighters from Al Qaeda’s Yemeni affiliate moved closer from one side and Houthi fighters pushed closer from the other. About 100 American personnel, including Special Forces commandos, were reportedly stationed there before the evacuation.
Houthi leaders have said their drive to the south is a battle to root out the Islamic State and Al Qaeda, portraying both groups as instruments of a broad international conspiracy including Israel, Saudi Arabia as well as Washington.