Shiite Houthi fighters are seen in an armed vehicle in al-Bayda province, south of Sanaa, Yemen’s capital, on Tuesday. (AFP/Getty Images)

The United States will close its embassy in Yemen amid mounting concerns over deteriorating security in the country’s capital, where Shiite rebels have seized control of the government and ousted a critical U.S. counter­terrorism ally.

The State Department said late Tuesday the decision was made “due to the uncertain security situation in Sanaa,” the Yemeni capital. It said that embassy operations and staff had been “temporarily relocated” and that “we will explore options for a return . . . when the situation on the ground improves.”

Britain and France said Wednesday that they were closing their embassies as well, the Associated Press reported.

Deif Allah al-Shami, a member of the Houthi political bureau, said: “We really have no idea why these embassies are closing and we don’t have a comment, except that these embassies know that they are safe here and under no threat.”

A senior U.S. official said the State Department had formed a task force to oversee the departures of dozens of diplomatic officials from the embassy compound, which has also served as a base for the CIA and other U.S. spy agencies involved in operations against a dangerous al-Qaeda ­affiliate.

The embassy closure comes as the leader of the Houthi rebel movement denounced foreign governments’ plans to withdraw their diplomats and declared that the group would not be deterred from expanding its authority over the fractured country.

“We will not accept pressures. They are of no use,” Abdulmalik al-Houthi said during a speech broadcast on Yemeni television, according to published reports. Although he did not single out the United States, he said that those who harm “the interests of this country could see that their interests are also harmed.”

The threat underscored the stakes for the United States, which has relied extensively on cooperation from Yemen’s government in counterterrorism operations against al-Qaeda in the Arabian Peninsula (AQAP) — a group linked to bomb plots against the United States and a recent attack in Paris that targeted journalists at the satirical newspaper Charlie Hebdo.

“It’s been happening slowly over time,” said the official, who spoke on the condition of anonymity, citing the sensitivity of the issue. The State Department delayed any announcement on the closure because “we want to get people out of there safely,” the official said.

Although the pending closure was widely reported earlier Tuesday, a formal announcement was delayed because “we want to get people out of there safely,” said the official, who spoke on the condition of anonymity, citing the sensitivity of the issue.

The State Department statement said that the ambassador and staff “will continue to engage Yemenis and the international community to support Yemen’s political transition process.” In a separate warning to U.S. citizens to not to travel to Yemen, the department said all embassy and consular activities had been “relocated out of the country” but did not say where.

Since withdrawing last year from Libya, the U.S. diplomatic mission to that country has operated out of Malta.

The closure in Yemen represents a significant setback for the United States and the optimism it once harbored about the prospect for political reforms amid the Arab Spring. Instead, the United States has seen allies in the region dwindle and been forced at least temporarily to shutter three of its diplomatic posts, in Syria as well as Libya and now Yemen.

The move also compounds the damage to a relationship that President Obama has touted as a model for his administration’s approach to fighting terrorism.

The United States has spent hundreds of millions of dollars training and equipping Yemeni military and security units that have carried out ground raids against al-Qaeda militants. At the same time, the CIA and the U.S. Joint Special Operations Command have launched dozens of drone strikes against AQAP targets.

Those missions are now endangered by the expanding authority of a Houthi movement that is hostile toward al-Qaeda but whose leaders have also repeatedly denounced the United States and described drone strikes as a violation of Yemen’s sovereignty.

U.S. officials have insisted that drone operations will continue, and the CIA has carried out three airstrikes in recent weeks, including a strike that killed a senior AQAP operative who publicly praised the attacks that killed 17 people in Paris.

But the Obama administration is now in the position of carrying out lethal military operations in a country where it no longer has explicit permission to do so and where the cooperation that has been critical to locating AQAP targets appears increasingly endangered.

The CIA declined to comment on the recent strikes or on whether the agency will be forced to withdraw officers from Yemen because of the embassy closure. Pentagon officials said Tuesday that U.S. military personnel remain in the country.

“We still have Special Operations forces in Yemen,” said Rear Adm. John Kirby, the Pentagon press secretary. “And we are still capable inside Yemen of conducting counterterrorism operations.”

At the same time, Kirby acknowledged that U.S. forces in Yemen had been hamstrung since the Houthi takeover. “I’d say there’s no question as a result of the political instability in Yemen that our counterterrorism capabilities have been affected,” he said. “I couldn’t stand up here and tell you that they haven’t been.”

Another Pentagon spokesman, Maj. Brad Avots, said current training programs are focused on basic military skill sets, such as small-arms fire and medical rescues. He said the Defense Department had other forces in the region available to help the State Department at the embassy.

Pentagon officials declined to say how many U.S. troops remain in Yemen. In the past, U.S. Special Operations forces have worked primarily out of al-Anad air base in southern Yemen but have also helped to transport Yemeni security forces on missions throughout the country. Dozens of U.S. military personnel have also been assigned to several offices within the U.S. Embassy compound in Sanaa.

U.S. officials stopped short of describing the embassy closure as an evacuation and said that many diplomatic officials would probably depart aboard commercial aircraft. The administration first withdrew families and nonessential personnel from the country in September and late last month further reduced its diplomatic profile, with “routine consular services” available only to American citizens and only on an emergency basis.

Backed by Iran, the Houthis have for years fought against the Sunni-controlled regimes of Yemeni President Abed Rabbo Mansour Hadi — who resigned last month and remains under house arrest — and his predecessor, Ali Abdullah Saleh. Houthi militants moved beyond their stronghold in northern Yemen last year and by September had seized much of the capital.

Houthi leaders had not asserted control of the government, however, until last week, when they ordered the dissolution of Yemen’s parliament and declared the creation of a Revolutionary Committee.

An earlier version of this story incorrectly stated that the American Embassy in Yemen would remain guarded by U.S. military personnel following the departure of diplomats from the facility. U.S. military personnel in charge of protecting the embassy have all been relocated out of the country.

Karen DeYoung and Craig Whitlock in Washington and Hugh Naylor in Sanaa contributed to this report.