The UN rights chief on Tuesday demanded that Yemen's Houthi rebels "immediately and unconditionally" release detained UN staff and other aid workers, flatly rejecting allegations that they were part of a spy network.

The Houthis on Monday said the people they arrested last week were part of a US-Israeli spy network, adding that those held worked under "the cover of international organizations and UN agencies".

"I categorically reject the outrageous allegations against our staff, and am deeply worried about the conditions in which they are being held," Volker Turk said in a statement.

The UN and aid groups said late last week that the Iran-backed rebels had detained more than a dozen aid workers, many of them UN staff, in what appeared to be a coordinated move.

The Houthis, who are engaged in a long-running civil war that has triggered one of the world's worst humanitarian crises, did not specify how many people were arrested.

The rebels seized control of the capital Sanaa in September 2014, prompting a Saudi-led military intervention on behalf of the government the following March.

Providing a detailed breakdown on the UN side, Turk's statement said 13 of the world body's personnel had been detained, including six members of his own staff.

"I urge the de facto authorities in Sanaa, Yemen, to immediately and unconditionally release them, as well as other individuals detained in relation to their affiliation with the UN, international NGOs or other actors supporting humanitarian activities," he said.

"It is crucial that the de facto authorities ensure that those detained are treated with full respect for their human rights and dignity, and that they are able to contact their families," he said.

"Access to the detained colleagues by the United Nations must be granted as soon as possible."

As for non-UN staff detained, the Yemeni Mayyun Organization for Human Rights said at least 18 Yemeni aid workers were kidnapped in four rebel-held parts of the war-torn country.

The Houthis have kidnapped, arbitrarily detained and tortured hundreds of civilians, including United Nations and NGO workers, since the start of Yemen's conflict in 2014, according to Human Rights Watch.

The latest "detentions come in addition to two other UN Human Rights staff, who were already detained, one of whom has been detained since August 2023, and the other since November 2021," Turk said.

"Both of them have been held incommunicado, without any due process," he lamented, adding that "UNESCO also had two personnel detained prior to the latest detentions".

Turk pointed out that his office had been "working in Yemen since 2012, for the promotion and protection of the rights of all the people of Yemen, including through engagement with the de facto authorities".

"Any further targeting of human rights and humanitarian workers in Yemen must cease immediately," he said.