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 noted that 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 strongly condemned the arbitrary detentions of at least 11 UN personnel by the Houthis. He also condemned the forced entry into WFP and other UN premises, as well as the seizure of property.

The exact reasons behind the Houthis targeting UN staff remains unclear, though the organisation's personnel have been vital in providing humanitarian aid to millions of Yemenis suffering from a protracted crisis.

One previously detained individual reportedly died in custody earlier this year, according to Grundberg.

Furthermore, the Houthis have detained around 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 continues, Grundberg stated.

Such actions significantly hinder broader efforts to deliver assistance and advance peace in Yemen.

The latest arrests occur against a backdrop of increased tension following an Israeli attack that killed the Houthi prime minister and other high-ranking officials on Thursday.

This incident represents a major escalation as the Houthis have been responding to Israeli missile attacks in support of the Palestinians in Gaza.

Since ousting the internationally recognized government in 2015 during a civil war, the Iran-backed group has controlled much of northern Yemen, resulting in over 150,000 fatalities and a humanitarian disaster displacing nearly 4.8 million people, with half of the population requiring aid.