UN Secretary General Antonio Guterres has called for the "immediate and unconditional release" of 11 UN staff arrested by Houthi authorities in Yemen.

The workers were detained by security forces in raids on the World Food Programme (WFP), children's charity Unicef and the World Health Organization (WHO), news agencies and the UN said.

Sunday's raids took place in the capital, Sanaa, and port city of Hudaydah, UN Special Envoy for Yemen Hans Grundberg said.

Grundberg said the Houthis had already been holding 23 other UN workers arrested since 2021. 

Houthi authorities have not commented on Sunday's arrests.

In a statement on Sunday, Guterres said he "strongly condemn[s] the arbitrary detentions of at least 11 UN personnel" by the Houthis. He also condemned what he said was the forced entry into WFP and other UN premises, as well as the seizure of property.

It is not clear why the Houthis targeted UN workers. The organisation's staff and aid workers have provided a crucial lifeline for ordinary Yemenis whose country has for years been suffering one of the world's worst humanitarian crises.

One of those previously arrested died in detention earlier this year, Grundberg said.

The Houthis have also detained some 20 Yemeni employees of the US embassy for the past three years.

"Despite sustained engagement and assurances sought over the last year, the arbitrary detention of UN staff, NGO workers and civil society has continued," Grundberg said.

"These actions severely hinder broader efforts to deliver assistance and advance peace in Yemen."

Earlier this year, the UN suspended all movement in Houthi-held areas of Yemen after a number of its personnel were detained.

The latest arrests come amid heightened tension after an Israeli attack killed the Houthi prime minister and other high-ranking officials on Thursday.

The attack was the biggest blow to the Houthis by Israel since it started retaliating a year ago against missile attacks by Houthis. The movement has been firing at Israel since November 2023 in support of the Palestinians in Gaza, where Israel is fighting a war with Hamas.

The Iran-backed group has controlled the capital and the north-west of Yemen since ousting the internationally recognised government in 2015 during a civil war.

The fighting has reportedly left more than 150,000 people dead and triggered a humanitarian disaster. Some 4.8 million people have been displaced and 19.5 million - half of the population - are in need of some form of aid.