Another United Nations staff member has been arrested in Yemen, bringing the number of UN officials arrested in recent days to six, a UN spokesman said on Monday.

The arrest came after Houthi security forces stormed several United Nations offices in Yemen’s Sana’a on Sunday, the Stéphane Dujarricrepresentative of the UN secretary general.

He added that the Houthis seized IT and communications equipment and assets.

As reported by the news agency Reutersat least 59 UN staff are being held by the Houthis, according to Dujarric, who condemned the continued arrests. Some have been held for years, the UN said.

Hundreds of UN staff remain in Houthi-controlled areas of Yemen, including a small number of international staff, the Farhan Haq, deputy representative of the UN secretary general.

“Our immediate priority is the safety and well-being of all UN staff, including ensuring the release of colleagues who have been arbitrarily detained and those of our local partners who have also been affected, and preventing further detentions”Huck stated.

The Iran-allied Houthis have controlled most of Yemen, including the capital Sanaa, since seizing power in 2014 and early 2015.

According to the UN, the Houthis have taken measures that they have made “increasingly difficult” for the organization to provide aid to those in need in Yemen.

Hajj said last week that the Houthis’ actions are forcing the UN to reassess how it operates in areas controlled by the group.