Sanaa, Yemen ¨C The U.N.¡¯s office in Yemen said Sunday that Houthi rebels were still holding 20 of its staff following their raid on the organization¡¯s building in Sanaa, Yemen¡¯s capital, a day earlier.

On Saturday, the U.N. office said Houthi security forces had made an ¡°unauthorized entry¡± into its compound.

¡°Five national staff and 15 international staff remain detained within the compound,¡± Jean Alam, spokesman for the U.N. coordinator in the country, said on Sunday.

The U.N. was in touch with the authorities in Sanaa, with the relevant member states and the government of Yemen ¡°to resolve this serious situation as swiftly as possible, end the detention of all personnel, and restore full control over its facilities in Sanaa,¡± he added.

Late on Sunday, a U.N. official, requesting anonymity, said that UNICEF¡¯s representative in Yemen, Peter Hawkins, was among those detained.

Two Houthi security sources confirmed that Hawkins was among those being held.

The rebels had already stormed U.N. offices in Sanaa on Aug. 31, detaining more than 11 employees, according to the U.N..

Those employees were suspected of spying for the United States and Israel, a senior Houthi official said at the time on condition of anonymity.

In a statement on Saturday, U.N. Secretary-General Antonio Guterres¡¯s spokesman, Stephane Dujarric, said, ¡°We will continue to call for an end to the arbitrary detention of 53 of our colleagues.¡±

He was responding to a televised address on Thursday by rebel leader Abdelmalek al-Huthi.

He claimed his forces had dismantled ¡°one of the most dangerous spy cells,¡± which he said was ¡°linked to humanitarian organizations such as the World Food Program and UNICEF.¡±

Dujarric described the accusations as ¡°dangerous and unacceptable.¡±

Saturday¡¯s raid came with dozens of U.N. personnel already arrested in recent months in areas controlled by the Iranian-backed group.

In mid-September, the U.N. humanitarian coordinator in Yemen was officially transferred from Sanaa ¡ª which is held by the Houthi rebels ¡ª to Aden, the interim capital of the internationally recognized government.

Since Aug. 31, 21 U.N. personnel have been arrested, adding to the 23 current and former members of international NGOs already detained, according to the U.N.

Ten years of civil war have plunged Yemen, one of the poorest countries in the Arabian Peninsula, into one of the worst humanitarian crises in the world, the U.N. says.