SANAA, Yemen — Shiite insurgents stormed Yemen’s presidential palace and besieged the leader’s residence Tuesday in a show of force that threatened to topple a government that has been a key American ally in the fight against al-Qaeda.

The attack by the Houthi rebel faction — believed to be backed by Iran — marked a major setback for President Abed Rabbo Mansour Hadi. While he apparently survived, and was nominally in charge, the rebel leader warned that the offensive “has no ceiling” if the president does not implement plans that include granting more power to the insurgents.

A government collapse could send the country into full-scale civil war, threatening a Syria-like disintegration that many fear could be exploited by radical groups like al-Qaeda. Yemen is home to the terror group’s most powerful branch, al-Qaeda in the Arabian Peninsula, or AQAP.

Hadi’s weakened position will likely spell trouble for Washington, which has relied heavily on the 69-year-old former general for cooperation in carrying out drone strikes that have targeted the al-Qaeda group. 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. They met little resistance from Yemen’s military, which has had a strained relationship with Hadi.

Tuesday’s assault brought Hadi’s government to the brink of collapse.

Yemen’s information minister, Nadia Sakkaf, wrote in an Arabic tweet that the “Yemeni president is being attacked by armed militias that want to overthrow” the government. She wrote from Sanaa that the presidential palace had been under siege since 3 p.m. “even though political talks are still ongoing.”

A government official, who spoke on condition of anonymity because of concern for his safety, said that Hadi was pinned down by an assault on his residence, about three miles from the palace.

In recent days, the Houthis have taken control of state-run media outlets and government buildings, including offices of the Yemeni intelligence service.

On Tuesday night, the rebel chief, Abdulmalik Houthi, delivered a long televised statement that stopped short of declaring a change of leadership. He leveled sweeping criticism against 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,” said the rebel leader. He called on the president to implement power-sharing agreements signed by the president 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 over 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.

In November, the U.N. Security Council imposed sanctions on Saleh and two Houthi leaders for threatening Yemen’s stability.

Hakim Almasmari, a Yemeni journalist, said there was little the Hadi could do to counter the Houthi advances. The president lacks support in the military because of his attempts to remove officers seen as loyal to Saleh, he said.

“Hadi has no loyalty with the army and that’s why you see the Houthis in power today,” Almasmari said, speaking by telephone from Sanaa.

Riad Kahwaji, chief executive of the Dubai-based Institute for Near East and Gulf Military Analysis, warned that the consequences of the current crisis could be severe.

“The world is very much occupied with what’s going on with Iraq and Syria, but we could find ourselves facing another civil war situation in Yemen, but one where there is a power vacuum for extremists,” he said.

The Houthis are opposed by the Sunni tribes, some of whom sympathize with AQAP. The crisis also risks causing the military to splinter into rival factions. Meanwhile, southern separatists have been agitating for several years to undo a 1990 pact that unified North and South Yemen, and may feel emboldened by the growing chaos.

Although the Houthis have battled with al-Qaeda-linked fighters before, the potential unraveling of central authority could offer breathing room for AQAP that could allow them to plan for attacks outside Yemen.

AQAP claimed responsibility for planning and funding the attack earlier this month at the French satirical newspaper Charlie Hebdo, which touched off days of terrorist violence that left 17 people dead.

In Washington, President Obama was monitoring the situation in Yemen, according to his senior adviser, Valerie Jarrett said.

“He obviously is in touch with the folks on the ground, our embassy, he’s getting regular updates from his national security team,” Jarrett told MSNBC.

The Houthi rebels issued seemingly contradictory statements about Tuesday’s violence. Before their leader spoke, a statement on a Houthi-run Web site claimed the group’s fighters took control of the presidential palace to avoid weapons looting amid the chaos. But the move appeared to be far more serious.

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.

Tuesday’s showdown came after days of turbulence. On Saturday, Houthi militants abducted Hadi’s chief of staff, Ahmed Awad bin Mubarak, who has been tasked with helping to draft a new constitution. The current version of the document includes measures that are opposed by the Houthis, such as a proposal to make Yemen a federal entity divided into six states.

The conflict intensified with clashes Monday in which at least nine people were killed. A government official, also speaking on condition of anonymity, said the country’s prime minister, Khaled Bahah, had been trapped inside another palace by the Shiite fighters since Monday.

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 ceasefire.

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.”