Jihadists from the group calling itself Islamic State who have seized control of parts of Iraq have turned centuries-old churches into places of detention.

For one, the Chaldean-run Church of the Immaculate Conception in the eastern part of Mosul was converted into a detention facility by the jihadists, a report by the Fides news agency said.

Citing a local news website, Fides said its sources said that IS fighters used the church to round up its detainees and hold them there.

Eyewitnesses said they saw a number of men that militants had blindfolded and handcuffed as they were brought into the church.

What they witnessed boosted speculation that one of the oldest churches in the area had been reduced to a prison facility by IS fighters who seized control of Mosul from the government in June.

Other sources said that the St. George's Monastery, which is also in Mosul, is being used by militants as a female detention center.

"The jihadists of the Caliphate have occupied the churches, including those which are very ancient," said Rebwar Audish Basa, procurator of the Order of St. Anthony sant'Ormisda of the Chaldeans in an interview with Fides.

"Among the concerns that plague us there is also that of those who fear that a possible military offensive for the liberation of Mosul could inevitably lead to consider churches as targets to hit, since they have become logistic bases of the jihadists," he said. "And of course, the destruction of old churches would be an irreparable damage and loss."

Both churches have a rich history within their walls.

The Church of the Immaculate Conception was considered to be the first Chaldean Catholic Church in Mosul, dating back as early as the 10th century. It served some 500,000 Assyrian Christians in the area.

The St. George's Monastery was built in the 17th century.

Sources told Fides they feared for the female detainees at St. George's as their captors might take advantage of them and force themselves on the prisoners, according to the news agency.