Houthi militants guard the house of Ali Haidar, a Houthi leader, destroyed by a Saudi-led airstrike in Sanaa, Yemen, on May 29. (Khaled Abdullah/Reuters)

The rebel group that has seized power in Yemen has taken at least four U.S. citizens prisoner, according to U.S. officials who said that efforts to secure the Americans’ release have faltered.

One of the prisoners had been cleared for release in recent days only to have that decision reversed by members of the Houthi rebellion that toppled the U.S.-backed government earlier this year and now controls most levers of power in Yemen.

The Americans are believed to be held at a prison in Sanaa, the Yemeni capital, which has been bombed repeatedly as part of an air campaign led by Saudi Arabia aimed at dislodging the Houthis from power. The United States has provided intelligence support to that operation.

The detention of the Americans has complicated U.S. efforts to navigate the chaotic aftermath of the Houthi takeover, which displaced a government that had cooperated extensively with the United States on drone strikes and other counterterrorism operations against a dangerous al-Qaeda affiliate in the country.

U.S. officials said three of the prisoners worked in private­sector jobs and that a fourth, whose occupation is unknown, has dual U.S.-Yemeni citizenship. The officials said none of the four were employees of the U.S. government.

The Washington Post is withholding some details about the prisoners at the request of U.S. officials and relatives who cited concerns for their safety.

A fifth U.S. citizen, Sharif Mobley, is also in Houthi custody, in connection with terrorism­related charges brought against him by the previous government more than five years ago. Mobley’s incarceration has been previously reported.

The recently detained prisoners are among dozens of U.S. citizens who were either unable to leave Yemen or chose to remain in the country after the U.S. government closed its embassy in February and began pulling out its employees and U.S. military personnel.

Details of the Americans’ detention remain murky, including where they are being held and whether they are together. In part this lack of detail is because there is little if any direct exchange of information between the United States and the Houthi movement, which has frequently employed chants of “Death to America.”

With no formal contact, U.S. officials said efforts to secure the prisoners’ release have gone mainly through intermediaries, including humanitarian groups that continue to have a presence in Sanaa.

U.S. officials said there is no indication that the prisoners have been physically harmed or are being treated as hostages. Still, they expressed concern about the well-being of one of the prisoners, who began to behave erratically in recent days as efforts to arrange for his departure unraveled.

The prisoner was initially detained “because he overstayed his visa,” a senior U.S. official said, speaking on the condition of anonymity. But Houthi leaders early this week cleared him for release, with flights arranged by the International Organization for Migration.

The Houthis abruptly withdrew that travel authorization on May 27, however, and accused the prisoner of having traveled without permission to “sensitive” regions in the country, the U.S. official said. Among those locations, the official said, was Abyan province in southern Yemen, which has been a stronghold for al-Qaeda fighters who are adversaries of the Houthis.

After being returned to a Yemeni prison, the American “acted as if he were mentally unstable” and removed his clothes, the U.S. official said, citing reports from sources in the Yemeni capital.

A member of the prisoner’s family declined to comment when reached by phone on Friday.

The Houthis are part of a Shiite sect that receives backing from the government of Iran. The United States and Iran are engaged in tense negotiations over Tehran’s nuclear program, and U.S. officials have denounced Iran’s detention and trial of Washington Post reporter Jason Rezaian. But U.S. officials said there have been no contacts with Iran over the Americans being held in Yemen.

Saudi airstrikes have reportedly killed more than 1,600 people in Yemen over the past several months.

In telephone conversations with his lawyers, Mobley has said he is being held in an area of the capital that has been targeted repeatedly. The site appears to have been struck again this week, killing 40 people, according to a statement released Friday by the Reprieve organization, which has represented Mobley.

“This raises some pretty disturbing questions about U.S. support for the bombing campaign,” said Namir Shabibi, a Reprieve official. Shabibi said that Reprieve has repeatedly asked U.S. officials to persuade the Saudis to avoid bombing Mobley’s location, but that “given the established Saudi record of hitting the same spot multiple times, there’s a high risk Sharif will be hit again.”

Julie Tate contributed to this report.