CAIRO -- Yemen's Houthi rebels hold dozens of women without bringing them to trial or charging them with a crime, often torturing the detainees and blackmailing their families, activists said on Thursday.

The allegations were first raised over the weekend by the Yemen Organization for Combating Human Trafficking, based in the capital, Sanaa. The group's founder, Nabil Fadel, told The Associated Press that he received information from families, former female detainees, and other sources showing that over the past months, the Houthis have been rounding up women over allegations of prostitution and collaboration with the Saudi-led coalition, which is at war with the rebels.

The rebel-run Interior Ministry responded Monday by saying the allegations were rumors from the "mouthpieces of the mercenaries" that are "tarnishing the image of security apparatus." It also denied the existence of secret prisons and illegal and arbitrary detentions and vowed to prosecute those behind the reports.

A Yemeni rights lawyer on Thursday told the AP the women were rounded up from cafes and parks the past months. Speaking on condition of anonymity for fears for personal safety, he said their families are searching for their missing daughters.

The Yemeni anti-trafficking group said it obtained new information showing that the rebels were carrying out atrocities such as "abuse, torture, and forced disappearances of women and girls in secret and illegal prisons."

The rebel leader, Abdul-Malek al-Houthi, had recently warned in televised speeches about a so-called "soft war" against the Houthis by their enemies, pointing to their "corrupt morals and sins."

Fadel, of the anti-trafficking group, said the arrests started after the Houthi appointment a year ago of Sultan Zabin as head of the Sanaa criminal investigation division.

Zabin promptly launched a crackdown on prostitution and smuggling. Women who had been rounded up in the crackdown and subsequently granted release were sent to secret detentions in villas across the Yemeni capital, instead of being set free.

An AP investigation last month showed that thousands of Yemenis have been imprisoned by the Houthi militia during the four years of Yemen's grinding civil war.

Many of them suffered extreme torture — being smashed in their faces with batons, hung from chains by their wrists or genitals for weeks at a time, and scorched with acid.

The revelations about women detainees come as representatives of Yemen's warring sides are in Jordan for talks on implementing a prisoners exchange deal agreed to in Sweden last month.

Yemen plunged into civil war in 2014, when the rebels captured the capital, Sanaa. A Saudi-led coalition intervened a year later, fighting alongside government troops.

In Sweden, the two sides agreed to confidence-building measures, including an exchange of thousands of prisoners. But the implementation of that has been slow marred by violence.