SANAA, Yemen — Shiite insurgents tightened their grip on Yemen’s capital Wednesday, seizing control of a missile base and keeping the president as a virtual hostage in a showdown threatening a key American ally in the fight against al-Qaeda.
Days of fast-moving advances by the Houthi rebel faction — believed to be backed by Iran — has left the Western-backed government of President Abed Rabbo Mansour Hadi backed into a corner with rapidly diminishing options.
Just hours after storming the presidential palace on Tuesday, the Houthi leader gave what amounted to an ultimatum: Hadi can either move ahead with reforms that include giving rebels more power or risk intensified attacks that could topple his government.
The brinksmanship and uncertainty have pushed Yemen closer to a full-scale political breakdown that could resonate deeply in Washington and among its key regional allies, including neighboring Saudi Arabia.
Washington depends on Hadi’s government as a critical partner against al-Qaeda’s branch in Yemen, which is considered one of the most active in recruitment and waging potential attacks. The group, known as al-Qaeda in the Arabian Peninsula (AQAP), claimed responsibility for a role in the deadliest attack amid the Paris bloodshed earlier this month.
Greater chaos in Yemen could be a serious setback to U.S.-led pressures against AQAP, including drone strikes. It also could open another sectarian battleground in the Middle East between Sunni-led al-Qaeda and the rival Shiite rebels.
A closely watched barometer of U.S. worries is whether the Obama administration keeps open the embassy in Yemen’s capital, Sanaa. There were no immediately signs from Washington of a full diplomatic withdrawal.
Even as Houthi fighters consolidated control in Sanaa, the president remained pinned down in his besieged residence. One presidential adviser, speaking on condition of anonymity to the Associated Press, described Hadi as a “captive.”
Rebels, meanwhile, added to their growing list of captured prizes. Houthi units easily took control of a missile base overlooking the city, the AP reported. The fighters apparently faced no resistance in another possible sign of deep divisions and disarray in the military ranks.
In protest of the Houthi actions, authorities closed the airport in the southern port city of Aden on Wednesday “until further notice,” Tarek Abdu, the head of the air facility, said in comments reported by the AP. The city’s seaport, a key lifeline for landlocked Sanaa, also was shut.
The Houthis have been vocal critics of the U.S. government. But it was not immediately clear whether the rebels would force the Yemeni president to suspend the strikes, since the Houthis also consider al-Qaeda an enemy.
The Houthis, followers of the Zaydi branch of Shiite Islam, are based in the northern Saada province but swept into the capital in September. The collapse of cease-fire talks on Monday then triggered a series of blitz-style rebel attacks at the heart of power.
Fighters stormed the presidential palace and surrounded Hadi in his residence, about three miles away.
Hours later, the rebel chief, Abdulmalik Houthi, delivered a long televised statement that stopped short of declaring a change of leadership. He leveled sweeping criticism at Hadi for alleged corruption and for failing to unite a country beset by years of unrest and a growing water shortage.
He demanded talks that could leave Hadi in charge — if barely.
“All options are open,” the rebel leader said. He called on the president to implement power-sharing agreements signed by Hadi and the Houthis in September.
The Houthis have mounted intermittent rebellions against the government since 2004 over what they say is discrimination. Zaydis form nearly a third of Yemen’s population of 24 million, which is majority Sunni Muslim.
Ali Abdullah Saleh, the former president, who is also a Zaydi, was able to remain in power for more than three decades, in part because of his ability to cultivate ties with Sunni officials and tribal leaders. He was forced out of office in 2012 by a popular uprising inspired by the Arab Spring. His departure led to Hadi winning a single-candidate election for the presidency.
Many in Yemen accuse Saleh of using his ties with the military to undermine the current president. They say he has conspired with the Houthis, who have been steadily advancing southward and now control nine provincial capitals.
The Houthis are opposed by the Sunni tribes, some of whom sympathize with AQAP. The crisis also risks splintering the military into rival factions. Meanwhile, southern separatists have been agitating for several years to undo a 1990 pact that unified North and South Yemen, and they may feel emboldened by the growing chaos.
Although the Houthis have battled with al-Qaeda-linked fighters before, the potential unraveling of central authority may offer breathing room for AQAP, allowing the group to plan attacks outside Yemen.
AQAP claimed responsibility for planning and funding the attack early this month at the French satirical newspaper Charlie Hebdo, the start of three days of terrorist violence in which three gunmen killed a total of 17 people. The assailants were killed in police raids Jan. 9.
Sunni Arab nations, including neighboring Saudi Arabia, accuse the Houthis of being a proxy for Shiite power Iran. The Houthis deny this and say they seek to root out corruption.
The U.N. Security Council on Tuesday called an emergency meeting after rebels seized the presidential palace, and issued a statement condemning the violence and calling for a cease-fire.
The statement, approved unanimously by the council’s 15 members, “underlined” that Hadi is “the legitimate authority based on election results.”
The Security Council said Yemenis “must stand with President Hadi” and his government in order to “keep the country on track to stability and security.”
Brian Murphy in Washington contributed to this report.