Yemen's Huthi rebels on Monday said they had arrested a "spy network" operating under the cover of humanitarian organisations, after aid workers including 11 UN staff were held last week.
The Iran-backed group claimed the network was linked to the CIA and had been carrying out "espionage" activities in Yemen for years, initially through the United States embassy before it suspended operations in Sanaa in 2015.
"An American-Israeli spy network was arrested," the Iran-backed group's security wing announced in a statement, saying those held worked under "the cover of international organisations and UN agencies".
The Huthis, 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 United Nations on Friday said 11 of its staff were among aid workers abducted by the Huthis in several rebel-run parts of Yemen, including six members of the Office of the High Commissioner for Human Rights.
Yemen's internationally recognised government said "dozens of employees" of UN agencies and other international organisations were detained as part of a "massive abduction campaign".
The Yemeni Mayyun Organization for Human Rights said at least 18 were held in "simultaneous" arrests that took place in the capital Sanaa, the key port of Hodeida, Amran and Saada, the rebels' traditional stronghold.
The Huthis have disappeared, 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.
A 'pretext'
Monday's announcement imperils vital humanitarian operations in rebel-run parts of the Arabian Peninsula's poorest country, where more than half of the population is dependent on aid.
Mohammed Albasha, senior Middle East analyst for the Navanti Group consultancy, called the espionage charges a "pretext", saying the move aims to "systematically eliminate non-governmental organisations in Yemen".
The arrests aim to force "foreign organisations or members of the international community seeking to conduct, fund, or implement projects inside the country" to deal exclusively with Huthi-run bodies, he said in a commentary published ahead of the rebels' announcement.
The Huthis seized control of Sanaa in September 2014, prompting a Saudi-led military intervention on behalf of the government the following March.
Since November, they have launched scores of drone and missile strikes targeting ships in the Red Sea and the Gulf of Aden, a campaign they say is intended to signal solidarity with Palestinians over the Gaza war.
In May, the Huthis also said they had exposed a "spy" network aiding the United States and Israel and arrested suspected members. A report by the Huthi-run Saba news agency did not specify the number of arrests, but unverified images published by Saba showed at least 18 people.