Islamic State of Iraq and Syria militants hammered, bulldozed and ultimately blew up parts of the ancient Iraqi Assyrian city of Nimrud, destroying a site dating back to the 13th century BC, an online militant video purportedly shows.

The destruction at Nimrud, located near the militant-held city of Mosul, came amid other attacks on antiquities carried out by the group now holding a third of Iraq and neighbouring Syria in its self-declared caliphate. The attacks have horrified archaeologists and UN Secretary-General Ban Ki-moon, who last month called the destruction at Nimrud "a war crime."

The seven-minute video, posted late Saturday, shows bearded militants using sledgehammers, jackhammers and saws to take down huge alabaster reliefs depicting Assyrian kings and deities. A bulldozer brings down walls, while militants fill barrels with explosives and later destroy three separate areas of the site in massive explosions.

"God has honoured us in the Islamic State to remove all of these idols and statues worshipped instead of Allah in the past days," one militant says in the video. Another militant vows that "whenever we seize a piece of land, we will remove signs of idolatry and spread monotheism."

The militants have been destroying ancient relics they say promote idolatry that violate their fundamentalist interpretation of Islamic law, including the ancient Iraqi city of Hatra, a UNESCO World Heritage site. Authorities also believe they've sold others on the black market to fund their atrocities.

Some of the figures in the video released Saturday at Nimrud appeared to have rebar, ribbed bars of steels designed to reinforce concrete that are a technique of modern building. An Iraqi Antiquities Ministry official, speaking Sunday on condition of anonymity as he wasn't authorized to talk to journalists, said all the items at Nimrud were authentic. In March, both Iraqi and United Nations officials warned the site had been looted and damaged.

The video conformed to other Associated Press reporting about the militants' attack.

Assyrians first rose around 2,500 BC

The Assyrians first rose around 2,500 BC and at one point ruled over a realm stretching from the Mediterranean coast to what is present-day Iran. They left dozens of palaces and temples decorated with huge reliefs mainly depicting their kings' military campaigns and conquests, hunting lions and making sacrifices to the gods. Their main hallmark was the colossal winged man-headed lions or bulls, protective deities put at the entrances of palaces and temples weighing about 10-30 tons each.

Located on the eastern side of the Tigris River, Nimrud, or Kalhu, was founded in the 13th century BC During the reign of King Ashurnasirpal II, Nimrud served as the second capital for Assyrian Empire. Other Assyrian capitals were Ashur, Dur Sharrukin and Ninevah.

Excavations at Nimrud were first started by the British traveller and archaeologist Austen Henry Layard from 1845 to 1851, followed by other foreign and local excavation missions.

The city is surrounded by a four-side wall measuring eight kilometres long. Among the ruins are the grand palace of Ashurnasirpal II, as well as the temples of Nabu, the god of writing and the arts, and other temples.

Among the most significant discoveries at Nimrud were four tombs of royal women. There, a collection of 613 pieces of gold jewelry and precious stones were unearthed. They survived the looting of the Iraqi National Museum that followed the U.S.-led invasion in 2003 as they were kept in a vault at the Central Bank of Iraq building by Saddam Hussein's government.