At least nine executives of of UN agencies from Yemen were arrested from the Houthi rebels under unspecified circumstances, authorities said Friday, according to the Associated Press (AP). The news agency adds that others working in aid groups are also likely to have been arrested.

Regional officials, speaking to the AP on condition of anonymity because they were not authorized to brief reporters, confirmed the arrests of the UN officials.

To those who are detained including United Nations human rights officialsof development program, the World Food Program and one who works for the office of his special envoy, the officials said. The wife of one of the prisoners is also being held.

According to the AP report, the Mayyun human rights group, which similarly identified the detained UN staff, named other aid groups whose workers were detained by the Houthis in four provinces. These organizations were initially unaware of the arrests.

“We unequivocally condemn this dangerous escalation, which constitutes a violation of the privileges and immunities of United Nations officials granted to them under international law, and which we consider to be oppressive, totalitarian, extortionate practices for political and economic gain”the organization said in a statement.

Activists, lawyers and others have also launched an open online campaign, calling on the Houthis to immediately release the prisoners because if they don’t, “contributes to the country’s isolation from the world.”

The AP notes that it is not clear what prompted the arrests. However, the news agency reports, this comes as the Houthis face problems with having liquidity to prop up the economy in the areas they hold – signaled by their move to introduce a new currency, the rial.

Yemen’s government-in-exile in Aden and other countries criticized the move as a Houthi turn to counterfeiting. The Aden authorities also demanded that all banks move their headquarters there.