Fighters of the Islamic State in Iraq and Syria (ISIS) have looted and bulldozed the ancient Assyrian city of Nimrud, the Iraqi government said, in their latest assault on some of the world's greatest archaeological and cultural treasures.

A statement from Iraq's Ministry of Tourism and Antiquities didn't elaborate on the extent of the damage, saying only that the group continues to "defy the will of the world and the feelings of humanity" with this latest act, which came after an attack on the Mosul museum just days earlier.

The UN cultural agency UNESCO condemned ISIS's actions as "cultural cleansing" and a war crime. In Paris, UNESCO chief Irina Bokova appealed in a statement to people around the world — "especially youth" — to protect "the heritage of the whole of humanity."

Bokova denounced "this cultural chaos" and said she had alerted both UN Secretary-General Ban Ki-moon and the prosecutor of the International Criminal Court.

"The deliberate destruction of cultural heritage constitutes a war crime," she said. "I call on all political and religious leaders in the region to stand up and remind everyone that there is absolutely no political or religious justification for the destruction of humanity's cultural heritage."

Nimrud, about 30 kilometres south of Mosul, was built around 1250 BC. Four centuries later it became capital of the neo-Assyrian empire — at the time the most powerful state on earth, extending to modern-day Egypt, Turkey and Iran.

I call on all political and religious leaders in the region to stand up and remind everyone that there is absolutely no political or religious justification for the destruction of humanity's cultural heritage. - Irina Bokova, UNESCO chief

Many of its most famous surviving monuments were removed years ago by archaeologists, including colossal Winged Bulls which are now in London's British Museum and hundreds of precious stones and pieces of gold which were moved to Baghdad.

The late 1980s discovery of treasures in Nimrud's royal tombs was one of the 20th century's most significant archaeological finds. After Iraq was invaded by the U.S. in 2003, archaeologists were relieved when they were found hidden in the country's central bank — in a secret vault-inside-a-vault submerged in sewage water.

'Cradle of Western civilization'

"It's really called the cradle of Western civilization, that's why this particular loss is so devastating," said Suzanne Bott, the heritage conservation project director for Iraq and Afghanistan in the University of Arizona's College of Architecture, Planning and Archaeology.

Many of the most valuable historical treasures found in Nimrud were moved to museums in Baghdad and England, but it's difficult to estimate the value of the artefacts destroyed by ISIS in the ancient city. (Radu Sigheti/Reuters) "What was left on site was stunning in the information it was able to convey about ancient life ... People have compared it to King Tut's tomb," she said.

Archaeologists have compared the assault on Iraq's cultural history to the Taliban's destruction of the Bamiyan Buddhas in Afghanistan in 2001. But the damage wreaked by ISIS, not just to ancient monuments but also on some Muslim places of worship, has been swift, relentless and more wide-ranging.

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 civilization."

Battle for Tikrit

On Friday Iraqi forces pressed their offensive against ISIS in Tikrit, expecting to reach the outskirts of the militant-held hometown of Saddam Hussein.

The battle to wrest Tikrit from ISIS is a major test for the Iraqi forces and allied Shia militias fighting on their side.

The governor of Salahuddin, Raed al-Jabouri, said that Iraqi forces expect to reach Tikrit later today. He told The Associated Press they still have not made it to Tikrit's east airport as some reports have suggested.

Tikrit, 130 kilometres north of Baghdad, has been under the control of ISIS since June, when the Sunni militants made a lightning advance across northern Iraq, prompting Iraqi troops to flee and abandon their weapons.

On Monday, Iraqi security forces launched a large-scale operation in an effort to retake the city from the militant group, but the offensive was stalled somewhat, with military officials saying the militants strategically lined roads leading to the city with explosives and land mines.