Syrian rebels have discovered disturbing rooms in Iraq and Syria (ISIS) territory that suggest they have been used as torture cells specifically to rape their female hostages.

"We discovered an Islamic State group prison for women, with rooms for groups as well as solitary prisoners," Ibrahim al-Mohammed told a local news agency last Thursday.

"We found the worst, most violent torture tools, and may God curse them."

The supposed ISIS prison for women was located in the Syrian city of Manbij after it was recaptured by the Manbij Military Council, a faction of the U.S.-backed Kurdish-led Syrian Democratic Forces.

In one of the rooms, the words (in Arabic) "God, I have wronged myself, and no one forgives sins but you. Forgive me, you are forgiving and merciful," were written on the wall.

According to God Reports, the message appears to have been written by a Christian woman. The woman apparently wrote the faithful message in response to being detained by ISIS and possibly being used as a sex slave.

In an amateur video taken at the facility, the cells have been shown to have metal doors and mattresses, pillows and blankets inside. In addition, sexual stimulants, contraceptives and narcotics were also found inside the prison cells.

"We found various kinds of pills, including sexual stimulation, contraceptives and narcotic pills which were used by the Islamic State group for torture," rebel leader Omar Mazerli said.

"They have very strange ways. This place was very hard to find and it is well hidden, but we got here with God's will and the efforts of the Military Council."

ISIS torture cells have been discovered in other areas around Manbij. It is highly possible that more cells of this kind will be discovered as more.