The UN called for the "immediate and unconditional" release of its staff held in Yemen, which is suffering one of the world's worst humanitarian crises.

"Yesterday, the de facto authorities in Sanaa detained additional UN personnel working in areas under their control," the office of the resident United Nations coordinator for Yemen said.

It did not say how many were held in the latest swoop by the Yemeni Houthis, who have detained dozens of staff from UN and other humanitarian organisations, most since the middle of last year.

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.

The UN has now suspended "all official movements into and within" areas held by Houthis, the statement said.

"UN officials in Yemen are actively engaging with senior representatives of the de facto authorities, demanding the immediate and unconditional release of all detained UN personnel and partners," the statement added.

The detentions come after United States President Donald Trump ordered the Houthis placed back on the US list of foreign terrorist organisations.

Re-listing the Houthis will trigger a review of UN agencies and other NGOs working in Yemen that receive US funding, according to the executive order signed on Wednesday.

'Pressure Trump'
 
Mohammed al-Basha of the Basha Report, a US-based risk advisory, called the latest detentions "an expected reaction" to the "terrorist" designation.

"They assume that by detaining UN staff they're going to be able to pressure the international community to pressure the Trump administration."

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, pressing Israel to end its war on Gaza, have targeted Israel-linked shipping in the Red Sea and directly attacked Israel since the war's outbreak, prompting reprisal strikes from US, 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 UN-brokered ceasefire in 2022.

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

In June, the rebels detained 13 UN 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 organisations -- allegations emphatically rejected by the UN Human Rights Office.

Two other UN 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.