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Yemen's Iran-backed Houthi rebels have detained more than a dozen aid workers, including United Nations staff, in an apparently coordinated sweep, a diplomatic source and a Yemeni NGO said Friday.

At least 18 Yemeni aid workers were kidnapped in four rebel-held parts of the war-torn country, the Yemeni Mayyun Organization for Human Rights said, listing 10 workers from UN agencies.

A diplomatic source who spoke on condition of anonymity also told French news agency AFP that more than a dozen aid workers including UN staff were kidnapped on Thursday.

There was no immediate comment from the Houthis or the United Nations.

The abductions underline the perilous task facing aid workers in Yemen, whose long-running civil war has precipitated one of the world's worst humanitarian crises.

"The Houthi armed group raided the homes and kidnapped staff of the United Nations and other international organisations operating in four governorates under" their control, the Mayyun Organization said.

This "serious escalation... constitutes a violation of the privileges and immunities of United Nations personnel", it added, describing the abductions as "blackmail practises in order to obtain political and economic gains".

The "simultaneous" abductions took place in the capital Sanaa, the key port of Hodeida, Amran and Saada, the rebels' traditional stronghold, the aid group said.

"The Houthis' actions are undermining essential humanitarian work in Yemen at a time when the majority of Yemenis do not have adequate access to basic necessities like food and water," Niku Jafarnia, Yemen researcher at Human Rights Watch, told AFP.

The Houthis have kidnapped, arbitrarily detained, and tortured hundreds of civilians, including UN and NGO workers, since the start of Yemen's conflict in 2014.

Several aid workers have been killed or kidnapped throughout the conflict, forcing international agencies to temporarily suspend operations or pull out international staff as a security precaution.

Last year, the charity Save the Children suspended operations for 10 days in northern Yemen after a staff member died in detention in the rebel-held capital.

Also last year, a long-serving staffer with the UN World Food Programme was shot and killed in the southern city of Taez by unknown gunmen.

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