Yemen’s Iran-backed Houthi rebels said Monday they have detained a number of U.N. humanitarian workers on suspicion of spying, including two Jordanians released over the weekend.

In a press conference held to dispute U.N. reports of corruption, Houthi leader Abdel Mohsen Tawoos railed against foreign aid agencies. He accused U.N. relief organizations of funding and conspiring with intelligence services to “secretly target” Yemenis, along with importing expired drugs and withholding fuel shipments. He did not provide evidence to back his claims.

The Houthis were responding to U.N. allegations that the rebels have diverted donated food, medicine, fuel and money from desperate Yemenis during the country’s five-year civil war.

Tawoos did not reveal the identities of the foreign workers he claimed the Houthis were holding, except to say two Jordanians had been transferred to Sanaa. The Houthis captured the capital, along with much of northern Yemen, from the internationally recognized government in 2014.

Jordan’s Foreign Ministry confirmed two citizens conducting a humanitarian audit in Yemen were released and flown home Sunday.

The World Food Program said that none of its workers are being held by the Houthis. Other U.N. agencies could not immediately be reached for comment.

The reported detentions underscore the risks to humanitarian providers in the war-torn country, where millions of Yemenis depend on aid for survival.

Last week, U.N. deputy humanitarian chief Ursula Mueller warned the Security Council that attacks on aid workers have escalated in Yemen’s Houthi-controlled areas.

The spike in violence comes as momentum is building to resolve the conflict. Warring parties have strengthened a cease-fire in the key port of Hodeida, and a power-sharing deal between Yemeni separatists and the government has halted infighting in the country’s south.

Still, fatal attacks on civilians continue across the country. Earlier this month, the Houthis launched a drone and missile attack that killed eight people and destroyed a hospital run by Doctors Without Borders, an international medical charity.

After the Houthi takeover of the country’s northern provinces, a Saudi-led military coalition intervened in 2015 to restore the government of President Abed Rabbo Mansour Hadi.

The fighting has killed tens of thousands of people and created the world's worst humanitarian crisis, leaving millions suffering from food and medical care shortages and pushing the country to the brink of famine.