The United Nations has condemned as a "war crime" the Islamic State group's bulldozing of the ancient city of Nimrud, the jihadists' latest demolition of Iraqi cultural treasures.

After rampaging through Mosul's museum with sledgehammers and torching its library last month, IS "bulldozed" the nearby ruins of Nimrud on Thursday, the tourism and antiquities ministry said.

Antiquities officials said IS militants had moved trucks last week to the site, which overlooks the Tigris River, 30km southeast of their main hub of Mosul.

"Until now, we do not know to what extent it was destroyed," one official said.

Nimrud was the latest victim of what appears to be a systematic campaign by the jihadists to obliterate Iraq's rich heritage.

"I'm really devastated. But it was just a matter of time, now we're waiting for the video. It's sad," Abdulamir Hamdani, an Iraqi archaeologist from Stony Brook University in New York said of the propaganda film of the destruction that IS will likely release.

He said the site's guards were denied access to Nimrud, which was founded in the 13th century BC and was once considered the jewel of the Assyrian era.

Its stunning reliefs and colossal statues of winged bulls with human heads guarding palace gates filled the world's museums in the 19th century.

A collection of 613 pieces of gold jewelry, ornaments and precious stones discovered in a royal tomb in 1988 has been described as one of the greatest archaeological finds of the 20th century.

"Their plan is to destroy Iraqi heritage, one site at a time," said Hamdani.

"Hatra of course will be next," he added, referring to a 2,000-year-old UNESCO-listed site about 100km south of Mosul known for its beautifully preserved temples blending Hellenistic, Roman and Eastern influences.

Irina Bokova, the head of the UN's cultural body UNESCO, condemned the destruction of Nimrud "with the strongest force".

"We cannot stay silent. The deliberate destruction of cultural heritage constitutes a war crime, and I call on all political and religious leaders in the region to stand up against this new barbarity," she said Friday.

UNESCO has called for tougher action to protect the many heritage sites in one of the cradles of civilisation but little can be done in areas under jihadist control.

IS attempts to justify the destruction by saying the statues are idolatrous, but experts say the jihadists traffic antiquities to fund their self-proclaimed "caliphate" and only destroy the pieces that are too bulky to be smuggled.

Most of Nimrud's priceless artefacts had long been moved to museums, in Mosul,

Baghdad, Paris, London and elsewhere but some giant "lamassu" statues of winged bulls and reliefs were still on site.