ISIS posted a video on Thursday on Twitter showing men destroying ancient artifacts with sledgehammers. It is unclear exactly when the video was recorded.
A man in the video describes the artifacts as "idols" and says they must be removed.
"God create us to worship him," the man says. "Him only not some stones."
"Our prophet ordered us to remove all these statues as his followers did when they conquered nations," he said.
A man in the video says, “These ruins behind me are idols and statues that people used to worship in the past instead of Allah.”
The video also depicts the burning of what appears to be hundreds of books.
Earlier this month, the director general of the U.N. Educational, Scientific, and Cultural Organization (Unesco) said the destruction involved museums, libraries, and universities across Mosul.
The museums and libraries in Mosul house artifacts from the Assyrian and Akkadian Empires.
Mosul is currently under ISIS control and has approximately 1,800 registered archeological sites.
The extremists have taken control of four ancient cities — Ninevah, Kalhu, Dur Sharrukin, and Ashur — which were all at some point the capital of the Assyrian Empire.
According to reports, the Unesco World Heritage Site Hatra, which is estimated to have been built in the 3rd or 2nd century B.C., is in danger from ISIS militants.
The British Museum released a statement about the video stating its concern over reports that militants destroyed ancient artifacts in the Mosul Museum and the Nergal Gate Museum.
"We naturally deplore all such acts of vandalism and destruction of cultural heritage, and continue to monitor the situation to the best of our ability. In the absence of further information it is difficult to verify the details of those objects featured in the footage. We can confirm that none of the objects featured in this video are copies of originals at the British Museum."