"God, I have wronged myself, and no one forgives sins but you. Forgive me, you are forgiving and merciful."

This was the message in Arabic scribbled on the wall of a prison used by the Islamic State (ISIS) to detain and torture women who resisted orders to become sex slaves. The message appears to have been written by a Christian woman, according to God Reports.

The prison was discovered in the formerly ISIS-occupied Syrian city of Manbij after it was liberated by fighters from the Manbij Military Council, which is part of the U.S.-backed, Kurdish-led Syrian Democratic Forces (SDF), the Arab24 news agency reports.

"We discovered an Islamic State group prison for women, with rooms for groups as well as solitary prisoners," Syrian rebel fighter Ibrahim Al-Mohammed told Arab24. "We found the worst, most violent torture tools, and may God curse them."

A video taken at the facility shows cells with metal doors and mattresses, pillows and blankets inside.

Sexual stimulants, contraceptives and narcotics were also found inside the prison.

"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 told Arab24. "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."

He said the prison was used by the ISIS to detain women who resisted orders to become sex slaves.

The discovery of the exclusive ISIS prison for women came a few weeks after the SDF gained control of the city from ISIS after months of heavy fighting.

Thousands of ISIS prisoners were freed when the Syrian rebel fighters took over the city. Many rejoiced in the streets, with men cutting their beards and women burning their burkas to signify an end to harsh ISIS regulations. The jihadis had imposed strict compliance regarding clothing and facial hair.

The capture of Manbij is considered as the most significant defeat suffered by the ISIS in Syria in a year. Manbij lies on a key supply route between the Turkish border and the city of Raqqa, the ISIS' capital.

The city earned the nickname "little London" due to the number of British jihadist fighters who stayed there on their way to Raqqa, according to Sky News correspondent Alex Rossi.

U.S. officials said the capture of Manbij clears a path for anti-ISIS forces to recapture Raqqa, according to Sky News.