BAGHDAD - Islamic State extremists continued their campaign targeting cultural heritage sites in territories they control in northern Iraq, looting and damaging the ancient city of Hatra just one day after bulldozing the historic city of Nimrud, according to Iraqi government officials and local residents. The destruction in Hatra comes as the group fended off an Iraqi army offensive in Saddam Hussein's hometown of Tikrit and fought pitched battles in eastern Syria in an area populated by predominantly Christian villages.

Officials in the northern city of Mosul said Saturday that Islamic State extremists have begun demolishing Hatra, a move UNESCO described as "cultural cleansing."

An official with the Ministry of Tourism and Antiquities' archaeological division in Mosul told the Associated Press that multiple residents living near Hatra heard two large explosions Saturday morning, then reported seeing bulldozers begin demolishing the site. He spoke anonymously for fear of reprisal. Saeed Mamuzini, a Kurdish official from Mosul, told the AP that the militants had begun carrying away artifacts from Hatra as early as Thursday and on Saturday, began to destroy the 2,000-year-old city.

Hatra, 68 miles southwest of Mosul, was a large fortified city during the Parthian Empire and capital of the first Arab kingdom. A UNESCO world heritage site, Hatra is said to have withstood invasions by the Romans in A.D. 116 and 198 thanks to its high, thick walls reinforced by towers. The ancient trading center spanned four miles in circumference and was supported by more than 160 towers. At its heart are a series of temples with a grand temple at the center - a structure supported by columns that once rose to 100 feet.

"The destruction of Hatra marks a turning point in the appalling strategy of cultural cleansing underway in Iraq," said Irina Bokova, the director-general of UNESCO, and Abdulaziz Othman Altwaijri, director-general of the Islamic Educational, Scientific and Cultural Organization (ISESCO) in a joint statement.

"With this latest act of barbarism against Hatra, [Islamic State] shows the contempt in which it holds the history and heritage of Arab people."

Meanwhile in Syria, Islamic State militants attacked a string of predominantly Christian villages on Saturday, touching off heavy clashes with Kurdish militiamen and their local allies, activists said.

The attack began around dawn and targeted at least three villages near the town of Tal Tamr along the Khabur River in Hassakeh province. Islamic State, also called ISIS and ISIL, kidnapped more than 220 Assyrian Christians from the same area last month after overrunning several farming communities on the southern bank of the river.