WASHINGTON 

The UK's envoy to the UN said Thursday that her country and 39 UN member states are demanding the "immediate and unconditional" release of UN staff detained by the Houthis. 

"We strongly condemn the latest detentions by the Houthis, since 7 June, of United Nations personnel and staff working for or who have previously worked for international and national non-governmental organisations and diplomatic missions," Barbara Woodward said at a news conference as she read a joint statement by the countries.

Woodward urged the Houthis to ensure the safety and security of humanitarian and UN personnel and said: "We are gravely concerned at the significant and rapid deterioration of the humanitarian situation in Yemen and underline that it is essential for humanitarian actors to have unimpeded access to civilian populations."

She said that the group of nations is also "deeply concerned" at the risk to delivery of essential humanitarian assistance and reiterated their demand to respect international humanitarian law with regard to the "safe, rapid and unimpeded" access for all humanitarian actors to ensure that humanitarian assistance can reach the most vulnerable in Yemen.

The countries reaffirmed their "strong commitment" to the unity, sovereignty, independence and territorial integrity of Yemen, and their commitment to stand by the people of Yemen.

UN Special Envoy for Yemen Hans Grundberg said Thursday that last week, 13 UN personnel, in addition to five staff members of international NGOs and many more from national NGOs and civil society were arbitrarily detained by the Ansar Allah group.

"I urge Ansar Allah to respect the rights of Yemenis under international law and release all UN and NGO personnel immediately and unconditionally and to refrain from the arbitrary detention of civilians," said Grundberg.

Yemen has been beset by violence and chaos since 2014 when Houthi rebels overran much of the country, including the capital, Sanaa. The crisis escalated in 2015 when a Saudi-led military coalition launched a devastating air campaign aimed at rolling back Houthi territorial gains.

Tens of thousands of Yemenis, including numerous civilians, are believed to have been killed in the conflict, while 14 million are at risk of starvation, according to the UN.