The ancient city of Nimrud is the latest target of Islamic militants now ravaging the cultural treasures of Iraq.

In a brief statement, the country's Ministry of Tourism and Antiquities reported that ISIS had bulldozed the site, but it offered no information on the extent of the damage.

Spreading along the east bank of the Tigris River south of the modern city of Mosul, the site was one of four consecutive capitals of the Assyrian Empire.

"Nimrud is the modern name," says Nicholas Postgate, a professor of Assyriology at the University of Cambridge. "The ancient name was Kalhu. It's mentioned in the Bible, under the spelling 'Calah.' "

A city had already taken shape at this location by 1400 B.C., but in the early ninth century B.C. King Ashurnasirpal II made it into his new administrative capital, adding a five-mile-long wall, a monumental stepped tower called a ziggurat, new temples, and a large palace covered in elaborate decorations.

It's those royal decorations that are of greatest concern. They consist of large stone panels of intricately carved reliefs that line the base of the building's mud-brick walls. In boldly delineated detail, the panels show military campaigns, conquered peoples offering tribute to the king, ritual ceremonies undertaken by the king (sometimes alongside an ornamental, sacred tree), and many winged mythical figures known as geniis.

"I think there must be hundreds of meters of those reliefs," says Postgate. "Many of the rooms in the palace had them."

The city also had sculptures in the round—winged bulls and lions that flanked some of the gateways. These were similar to the figures that the Islamic militants destroyed just days ago at the ancient Assyrian site of Nineveh.

Archaeologists have also uncovered sculptures from the time of king Tiglath-Pileser III, who ruled between 745 and 727 B.C.

Assyrian Palace Is One of a Kind

"The attack is hugely significant and can't be underestimated," says John Curtis, president of the British Institute for the Study of Iraq. "This is the best preserved Assyrian site."

Nimrud was first properly excavated between 1845 and 1851 by Austen Henry Layard, an English politician and historian who exposed many of the sculptures from the time of Ashurnasirpal II. A number of those works of art were then transported to the British Museum, where they're on display today, and to other institutions in Europe and the United States.

But many more of the reliefs were left at the site—and were among what may have been lost in the recent attack.

From 1949 to 1957 Max Mallowan—mystery writer Agatha Christie's husband—excavated at the Nimrud palace and other buildings.

David Oates, a Mesopotamian archaeologist at the University of Cambridge, continued work at the site from 1958 to 1962. He focused on the military palace built by Ashurnasirpal II's son Shalmaneser III.

"That was a very large building, but it didn't have the stone reliefs that we're concentrating on at the moment—that's the stuff at risk," says Postgate.

A Polish expedition also worked at the site, exposing additional reliefs from the time of Ashurnasirpal II and Tiglath-Pileser III. Italian and British expeditions followed.

Finally, in the 1980s and 1990s, Iraqi archaeologists worked at the site.

"They actually had the most spectacular finds, which were the tombs of the queens of Assyria," says Postgate. "The tombs were absolutely stuffed with gold and carved ivory."

The Iraqis also continued to clear out the rooms of the main palace, exposing even more reliefs. To protect what they'd uncovered from the weather, they constructed a roof over part of the building.

Damage Unknown

The nature and extent of the damage inflicted by ISIS is unclear. "The militants may have bulldozed the excavated earth back into the palace, just to cover it up," Postgate explains. "Or they could have driven their machines straight into the walls."

In that case, the reliefs have likely been obliterated. "They were made of a type of gypsum called Mosul marble," says Curtis. "It's very fragile. I imagine if you hit a relief hard with a big digger, the whole thing would shatter."

Reports are also circulating about sculptures being loaded into a truck and driven away, perhaps with the misguided idea that they could be sold on the international black market.

"That's not going to be much use to them," says Postgate. "These reliefs are very big. A segment of a scene won't be very attractive, and there aren't any small sculptures."