Recently taken pictures show how Isis have destroyed two of the great gates in an ancient Assyrian citadel in northern Iraq.
These photographs seem to confirm the Nergal and Mashki gates in Nineveh have been bulldozed into rubble.
While the original gates date back over 2,500 years, those just destroyed were replicated in the mid-20th century, National Geographic reported.
Their destruction was confirmed by Dr Michael Danti, the co-director of the Cultural Heritage Initiative (CHI), who works alongside the US State Department to document destruction of cultural and religious sites in the region.
The archaeology professor from Boston University told National Geographic: "We can verify... that the Mashki and Nergal Gates have been destroyed."
He said the images were obtained from trusted sources inside the surrounding Isis-controlled-city of Mosul.
A bulldozer is clearly visible in the picture of the destroyed Nergal gate.
At the time these gates were built in 700 BC, Nineveh was at the height of its prominence as the largest city in the ancient world.
There are unconfirmed reports that the remaining gates left standing are currently being brought down.
Professor Danti said: "It's reasonable to assume that [Isis] is destroying other gates [in Nineveh], but we need to get more information before we can make those assertions.
"[Isis] is essentially sending a signal to the government of Iraq that if they continue with their offensive on Mosul they can expect a scorched earth policy from Islamic State, in both Mosul and the Iraq theatre, but also in Raqqa in the Syrian theatre."
Isis have already devastated Mosul museum in addition to destroying the city's libraries, obliterating manuscripts dating back centuries.
