At least nine 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 US-led coalition. Others working for aid groups also likely 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.

Regional officials, speaking to The Associated Press on condition of anonymity as they were not authorised to brief journalists, confirmed the UN detentions. Those held include staff from the United Nations human rights agency, its development program, the World Food Program and one working for the office of its special envoy, the officials said. The wife of one of those held is also detained.

The UN declined to immediately comment.

The Mayyun Organisation for Human Rights, which similarly identified the UN staffers held, named other aid groups whose employees were detained by the Houthis across four provinces the Houthis hold — Amran, Hodeida, Saada and Saana. Those groups did not immediately acknowledge the detentions.

“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 organisation said in a statement.

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.”

Yemen's Houthi rebels and their affiliated media organisations did not immediately acknowledge the detentions. However, the Iranian-backed rebels planned for weekly mass demonstrations after noon prayers Friday, when Houthi officials typically speak on their actions.

It's unclear what exactly sparked the detentions. However, it comes as the Houthis have faced issues with having enough currency to support the economy in areas they hold — something signalled by their move to introduce a new coin into the Yemeni currency, the riyal. Yemen's exiled government in Aden and other nations criticised the move as the Houthis turning to counterfeiting. Aden authorities also have demanded all banks move their headquarters there.

“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 US planned to further increase economic pressure on the Houthis by blocking their revenue sources, including a planned USD 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 US-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 are members of Islam's minority Shiite Zaydi sect, which ruled northern Yemen for 1,000 years until 1962.