Islamic State militants have started bulldozing the ancient Assyrian city of Nimrud in northern Iraq, the country's tourism and antiquities ministry has said.
In a statement on its Facebook page, the ministry said IS had "assaulted the historic city of Nimrud and bulldozed it with heavy vehicles".
The statement did not elaborate on the damage, saying only the group continues to "defy the will of the world and the feelings of humanity" with its actions.
Officials said the destruction began after noon prayers on Thursday and that trucks that may have been used to take away artefacts had also been spotted at the site.
"Until now, we do not know to what extent it was destroyed," the official said on condition of anonymity.
UNESCO has said the destruction, if true, "constitutes a war crime" and called on people around the world "especially youth" to protect "the heritage of the whole of humanity".
Built in the 13th century BC, Nimrud is located on the Tigris River just south of Iraq's second largest city, Mosul, which was captured by IS in June.
The extremists, who control a third of Iraq and Syria, have attacked other archaeological and religious sites, claiming they encourage people to abandon Islam.
Abdulamir Hamdani, an Iraqi archaeologist from Stony Brook University, said: "I'm sorry to say everybody was expecting this. Their plan is to destroy Iraqi heritage, one site at a time.
"Hatra, of course, will be next," he said, referring to a beautifully-preserved city in Nineveh that is more than 2,000 years old and is a UNESCO world heritage site.
The destruction at Nimrud came a week after IS released a video showing militants armed with sledgehammers and jackhammers smashing priceless ancient artefacts at the Mosul museum .
That attack sparked widespread condemnation, with some archaeologists and heritage experts comparing it with the 2001 demolition of the Bamiyan Buddhas in Afghanistan by the Taliban.
Last year, IS destroyed the Mosque of the Prophet Younis - or Jonah - and the Mosque of the Prophet Jirjis, two revered ancient shrines in Mosul.
The group also threatened to destroy the city's 850-year old Crooked Minaret, but local residents surrounded the structure, preventing the militants from approaching.
Iraq's national museum in Baghdad opened its doors to the public last week for the first time in 12 years in a move Prime Minister Haider al Abadi said was to defy efforts "to destroy the heritage of mankind and Iraq's civilisation".
IS has imposed a harsh and violent version of Islamic law in the territories it controls and has terrorised religious minorities.
A US-led coalition has launched a military campaign against the group, and this week Iraqi forces began an offensive to try to retake the city of Tikrit, on the main road linking Baghdad to Mosul.