Eleven Yemeni employees of United Nations agencies have been detained by Yemen's Houthi rebels under unclear circumstances, authorities said Friday, as the rebels face increasing financial pressure and airstrikes from a U.S.-led coalition. Others working for aid groups also have been taken.

The detentions come as the Houthis, who seized Yemen's capital nearly a decade ago and have been fighting a Saudi-led coalition since shortly after, have been targeting shipping throughout the Red Sea corridor over the Israel-Hamas war in the Gaza Strip.

But while gaining more attention internationally, the secretive group has cracked down at dissent at home, including recently sentencing 44 people to death.

U.N. spokesman Stéphane Dujarric in New York acknowledged 11 U.N. staffers had been taken.

“We are very concerned about these developments, and we’re actively seeking clarification from the Houthi de facto authorities regarding the circumstances of these detentions and most importantly, to ensure the immediate access to those U.N. personnel," he told journalists. "So I can further tell you that we’re pursuing all available channels to secure the safe and unconditional release of all of them as rapidly as possible.”

Of the 11, the U.N. said nine are men and two are women. Six worked for the U.N.'s human rights agency, while one apiece worked for its special envoy's office, its development arm, UNICEF, the World Food Program and UNESCO.

The Mayyun Organization for Human Rights, which also reported U.N. staffers were held, named other aid groups whose employees were detained by the Houthis across four provinces that the Houthis hold — Amran, Hodeida, Saada and Saana.

“We condemn in the strongest terms this dangerous escalation, which constitutes a violation of the privileges and immunities of United Nations employees granted to them under international law, and we consider it to be oppressive, totalitarian, blackmailing practices to obtain political and economic gains,” the organization said in a statement.

Save the Children, told the AP that it was “concerned of the whereabouts of one of our staff members in Yemen and doing everything we can to ensure his safety and well-being.” The group declined to elaborate.

CARE International also said one of its staffers had been detained without being given a reason.

“We are concerned about our colleague’s safety and are working to get more information in the coming hours and days,” said Sulafah al-Shami, a CARE spokeswoman. “Until then, we have extended our support to the family and share their hope for his speedy release.”

Other groups also are believed to have staff who were taken as well, though they did not acknowledge it publicly.

Activists, lawyers and others also began an open online letter, calling on the Houthis to immediately release those detained, because if they don't, it “helps isolate the country from the world.”

Human Rights Watch, quoting family members of those detained, said that “Houthi authorities have not revealed the locations of the people they detained or allowed them to communicate with their employers or families.”

“The Houthis should immediately release any U.N. employees and workers for other independent groups they have detained because of their human rights and humanitarian work and stop arbitrarily detaining and forcibly disappearing people," Human Rights Watch researcher Niku Jafarnia said.

Yemen's Houthi rebels and their affiliated media organizations didn't discuss the detentions, though military spokesman Brig. Gen. Yahya Saree claimed attacks Friday night on ships that hadn't been reported damage or been acknowledged by international authorities.

The U.S. military's Central Command said the Houthis launched four anti-ship ballistic missiles over the last day that caused no damage. Separately, U.S. forces destroyed two missiles, five drones and one patrol boat, it said, something not acknowledged by the rebels.

The Iranian-backed rebels also reported new U.S.-led airstrikes Friday hitting around the Red Sea port city of Hodeida and later in the capital, Sanaa. Several hit Hodeida's airport, the Houthi-controlled SABA news agency said, where the rebels are believed to have launched attacks previously targeting shipping in the region.

It's unclear what exactly sparked the detentions. Former employees of the U.S. Embassy in Sanaa, which shuttered in 2015, also have been detained and held by the Houthis.

However, it comes as the Houthis have faced issues with having enough currency to support the economy in areas they hold — something signaled by their move to introduce a new coin into the Yemeni currency, the riyal. Yemen’s exiled government in Aden and other nations criticized the move, saying the Houthis are turning to counterfeiting.

Aden authorities also have demanded all banks move their headquarters there as a means to stop the worst slide ever in the riyal's value and re-exert their control over the economy.

“Internal tensions and conflicts could spiral out of control and lead Yemen into complete economic collapse,” warned Yemeni journalist Mohammed Ali Thamer in an analysis published by the Carnegie Endowment for International Peace.

Bloomberg separately reported Thursday that the U.S. planned to further increase economic pressure on the Houthis by blocking their revenue sources, including a planned $1.5 billion Saudi payment to cover salaries for government employees in rebel-held territory.

The war in Yemen has killed more than 150,000 people, including fighters and civilians, and created one of the world’s worst humanitarian disasters, killing tens of thousands more. The Houthis' attacks on shipping have helped deflect attention from their problems at home and the stalemated war. But they've faced increasing casualties and damage from U.S.-led airstrikes targeting the group for months now.

Thousands have been imprisoned by the Houthis during the war. An AP investigation found some detainees were scorched with acid, forced to hang from their wrists for weeks at a time or were beaten with batons. Meanwhile, the Houthis have employed child soldiers and indiscriminately laid mines in the conflict.

The Houthis previously have detained four other U.N. staffers — two in 2021 and another two in 2023. The U.N.’s human rights agency in 2023 called those detentions a “profoundly alarming situation as it reveals a complete disregard for the rule of law.”