Islamic State fighters have looted and bulldozed the ancient Assyrian city of Nimrud in northern Iraq, the government and a local tribal source said.The destruction at Nimrud came a week after the radical Islamist militants released a video showing them destroying Assyrian era statues and sculptures in the city of Mosul, which they seized in June last year."Islamic State members came to the Nimrud archaeological city and looted the valuables in it and then they proceeded to level the site to the ground," the tribal source from near Mosul, where ancient Nimrud is located, told Reuters."There used to be statues and walls as well as a castle that Islamic State has destroyed completely."Iraq's Ministry of Tourism said the Islamic State militants were defying the world with their destruction of antiquities."They assaulted the ancient city of Nimrud and bulldozed it with heavy machinery, appropriating the archaeological attractions dating back 13 centuries BC," it said in a statement issued late on Thursday.