SANA, Yemen — Fighters from Al Qaeda captured the headquarters of a Yemeni Army brigade on Thursday, as the United Nations issued a dire new warning that Yemen was headed toward disintegration.
The developments were a setback for the Houthi militants who since their takeover are nominally in control of the military in Yemen, the poorest country in the Arab world.
They came a day after the United States, which has long relied on Yemen as a partner in fighting Al Qaeda, closed its embassy indefinitely because of security concerns and the absence of a functioning government. Other embassies were preparing to close as well.
A statement by a news media liaison to Al Qaeda in the Arabian Peninsula said that Qaeda fighters had completely captured the Yemeni National Army’s 19th Brigade after heavy fighting at its headquarters in the Baihan district of Shabwa Province, an important oil-producing area in southeastern Yemen.
At the United Nations Security Council, where members were meeting to discuss the developments in Yemen, Secretary-General Ban Ki-moon and his special representative to Yemen, Jamal Benomar, put the deterioration in stark terms.
“Let me be clear,” Mr. Ban told the Council. “Yemen is collapsing before our eyes. We cannot stand by and watch.”
Mr. Benomar, who spoke to Council members via videolink from the Yemeni capital Sana, said the nation was heading into an economic free-fall and that weeks of political instability were “conducive" to a strengthening of Al Qaeda in the country.
Security Council diplomats were discussing prospects for a draft resolution aimed at a political settlement to Yemen’s crisis. The British ambassador, Mark Lyall-Grant, told reporters at the United Nations that “we are looking for a resolution in the next few days.”
Al Qaeda in the Arabian Peninsula, the organization's Yemeni affiliate, is already considered the most dangerous to Western interests.
The American military has small units in Yemen who help to manage a campaign of drone strikes against Al Qaeda, as well as provide training for the Yemeni military.
The former head of security for the Interior Ministry in Baihan District, Col. Mubarak Abdullah, confirmed that the army’s 19th Brigade headquarters had fallen to the militants. He said he did not know how many of the brigade’s 4,000 soldiers had been taken prisoner. Colonel Abdullah resigned recently as security chief but is still working in the security office in the district.
The brigade commander was released after a tribal negotiation, Colonel Abdullah said. Yemeni officials said most of the soldiers were allowed to leave with their personal weapons, while the Qaeda militants kept the heavy equipment and artillery stored at the base.
Security officials in the area said 12 soldiers were killed in the attack but resistance had been ineffective. “Given the shaky situation in the country, the fighting and defensive spirit is no longer with the people because they know no one will save them,” an official said.
It was unclear if the army brigade had previously been under the direct control of the Houthi militants, whose authority in the south and east of the country is unclear and often disputed.
The Houthis effectively took control of the military and the Defense Ministry when the government collapsed on Jan. 22, with the resignation of the American-allied president, Abdu Rabbu Mansour Hadi, after Houthi forces besieged his home.
Western diplomats here said the Houthi military has been trying to extend its power in the oil-rich east and south of the country, but military units in the provinces have loyalties to a variety of power brokers, including Mr. Hadi, his predecessor, Ali Abdullah Saleh, who was ousted as a result of the Arab Spring uprisings in 2011 but remains a political power, and to southern political factions.
Militia units of the Houthis, who deeply oppose Al Qaeda, had been said by diplomats to be advancing in recent days on areas of Shabwa Province held by Qaeda fighters. The Al Qaeda statement about its attack on the 19th Brigade said it had moved in advance of Houthi plans to take control of that brigade.
Al Qaeda militants have sought to profit from Yemen’s political turmoil by depicting themselves as defenders of Sunni Muslims against Houthi forces, described by Al Qaeda as Iranian-backed Shiite apostates.
The Houthis are dominated by members of northern Yemen’s Zaydi sect, an offshoot of Shia Islam, who represent about a third of Yemen’s population, but they have many non-Zaydi supporters as well.
The Houthis swept to dominance in northern and much of central Yemen and the capital last September and had been negotiating with Mr. Hadi for a share of political power. After Mr. Hadi’s resignation, political opponents accused the Houthis of in effect staging a coup, although United Nations talks, mediated by Mr. Benomar, have continued in efforts to form a government acceptable to all factions.
While the Houthis had espoused an anti-American policy, and denounced drone strikes, they are also strong enemies of Al Qaeda and since coming to power have made no move to interfere with either drone strikes or American military counterterrorism operations in Yemen. American military officials have said those operations are continuing normally.
Several other Western nations announced plans to close their embassies in Sana this week in the wake of the American departure. Only local guards remained in the American complex, but Houthi officials pledged to maintain the security of the building until American officials decide to return. They also said that vehicles and weapons taken by the Houthis from departing American officials on Wednesday would be safeguarded.