Yemen's Houthi rebels have detained another seven UN employees, the United Nations chief said on Friday, their latest move to target aid workers.

Secretary-General Antonio Guterres called for the "immediate and unconditional" release of all aid staff held in Yemen, which is suffering one of the world's worst humanitarian crises.

"Their continued arbitrary detention is unacceptable," Guterres said in a statement, adding that the United Nations was working to secure the release of those being held.

The Iran-backed Houthis have detained dozens of staff from U.N. and other humanitarian organizations, most of them since the middle of last year.

Guterres said the "continued targeting of UN personnel and its partners negatively impacts our ability to assist millions of people in need in Yemen."

Reeling from a decade of war, Yemen is mired in a humanitarian catastrophe with more than 18 million people needing assistance and protection, according to the United Nations.

Following the latest detentions, the United Nations has suspended "all official movements into and within" areas held by Houthis, the office of the resident U.N. coordinator for Yemen said.

The detentions come after U.S. President Donald Trump ordered the Houthis placed back on the U.S. list of foreign terrorist organizations.

Relisting the Houthis will trigger a review of U.N. agencies and other nongovernmental organizations working in Yemen that receive U.S. funding, according to the executive order signed on Wednesday.

Mohammed al-Basha of the Basha Report, a U.S.-based risk advisor, called the latest detentions "an expected reaction" to the "terrorist" designation.

"They assume that by detaining U.N. staff they're going to be able to pressure the international community to pressure the Trump administration," he said.

No immediate comment was available from the Houthis, who seized the capital, Sanaa, in 2014 and rule large parts of the impoverished country.

The rebels, saying they are acting in solidarity with the Palestinians, have been attacking the Red Sea shipping route and firing on Israel since the outbreak of the Gaza war, prompting reprisal strikes from U.S., Israeli and British forces.

With a Gaza ceasefire starting last Sunday, the Houthis have made conciliatory moves, including releasing the 25-strong international crew of the Galaxy Leader, a cargo ship they seized in the Red Sea in November 2023.

The rebels have also promised to tone down the Red Sea attacks and have said they would stop targeting Israel if it sticks to the ceasefire.

The Houthis have been at war with a Saudi-led coalition since 2015, although hostilities have fallen sharply since a U.N.-brokered ceasefire in 2022.

Since the start of the war, the Houthis have kidnapped, arbitrarily detained and tortured hundreds of civilians, including U.N. and NGO workers, according to rights groups.

In June, the rebels detained 13 U.N. personnel, including six employees of the Human Rights Office, and more than 50 NGO staff plus an embassy staff member.

They claimed they had arrested "an American-Israeli spy network" operating under the cover of humanitarian organizations — allegations emphatically rejected by the U.N. Human Rights Office.

Two other U.N. human rights staff had already been detained since November 2021 and August 2023, respectively.

In early August, the Houthis stormed the UNHCR office, forced staff to hand over the keys, and seized documents and property, before returning it later that month.