BAGHDAD (Reuters) -- Iraqi soldiers began painstaking work on Monday to secure the remains of the 3,000-year-old city of Nimrud, a day after driving out Islamic State militants who overran and ransacked the ancient Assyrian capital. Nimrud's palace and temples, once at the heart of an empire which stretched across the Middle East, were razed by the ultra-hardline zealots after they swept through northern Iraq in 2014, destroying historic sites they declared idolatrous. Whether any treasures at Nimrud can be rescued will be hard to assess until archaeologists can get there. That will take time because of fears the militants have left bombs or even fighters concealed in tunnels among the ruins. "We are stepping up drone surveillance now over Nimrud to make sure no imminent threat still exists," a colonel in the Ninth Armoured Division told Reuters. He said troops were in control of the area, but had strict instructions to be "extremely cautious" and were staying out of Nimrud while remaining villages nearby were cleared. The drones overflying the area were unarmed. "We want to make sure we don't inflict even the least damage to buildings in Nimrud already damaged by the evil Daesh (Islamic State)," he said. Nimrud lies on the eastern bank of the Tigris river, 30 km (20 miles) south of Mosul where Iraqi soldiers are battling to crush Islamic State. Mosul is the largest city under the militants' control in Iraq and neighbouring Syria. The Sunni Islamist group still controls the remains of the ancient city of Nineveh, in central Mosul. Khorsabad, another Assyrian site northeast of Mosul, is close to the frontline of the current conflict. To the south, the 2,000-year-old desert city of Hatra, famed for its pillared temple blending Graeco-Roman and eastern styles, was also seized and damaged by the militants in 2014. The UN cultural agency UNESCO has condemned the destruction at Nimrud as a war crime and an attack on the world's shared heritage, pointing to ancient Mesopotamia's role as a cradle of civilisation where early urban centres flourished and cuneiform writing on clay was developed. "Liberation of ancient Iraqi archaeological sites from the control of the forces of dark and evil is a victory not only for Iraqis but for all humanity," Iraq's deputy culture minister Qais Hussain Rasheed told Reuters after the army announced it had taken Nimrud. NIMRUD DEVASTATION In neighbouring Syria, Islamic State was driven out of the city of Palmyra eight months ago, after dynamiting monuments including two temples and Palmyra's imposing triumphal arch. Syria's antiquities chief said much of Palmyra could be restored, but video footage of the destruction carried out by the militants in Nimrud, and a government report issued last year, suggest the devastation in Iraq is extensive. The report by Iraq's culture ministry said a carved wall panel was stolen from the northern palace at Nimrud in July 2014. Eight months later, far greater damage was inflicted. The militants destroyed 10 colossal statues of winged bulls, located at the palace gates and at the temple of Ishtar - goddess of love, war, sex and power - and Nabu - god of literature and wisdom. A month later in April 2015 "the gangs completely blew up the city and its ancient buildings" the report said. Video released by Islamic State supporters, purporting to show them at work in Nimrud, included footage of the militants using bulldozers and electric drills to tear down murals and statues. They also rigged up barrels full of explosives which they appeared to detonate at the site. As recently as in the last three months, according to satellite imagery, Islamic State appears to have bulldozed the ziggurat, or step tower, built by Ashurnasirpal II and Shalmaneser III in the 9th century BC. Pictures released by the American Schools of Oriental Research (ASOR) Cultural Heritages Initiative showed that between late August and early November the towering remains of the ziggurat had been steadily flattened. What appeared to be scars from bulldozer shovels were visible, and ASOR said there were also signs of machinery tracks at the temple of Ishtar. A picture which ASOR dated Nov. 4 did, however, show contours of the palace and two temples still visible, suggesting they were not entirely erased. "It's premature to talk about the extent of damage that has been caused by Daesh," a government archaeological source said. "After the sites have been completely secured by the army, then we will send the archaeology police to cordon off the sites," the source added. "Then we will send our crews to take images and write a detailed report on the damage." Nimrud was excavated by a series of archaeological missions since the 19th century including in the 1950s by British archaeologist Max Mallowan, who was accompanied by his wife, detective novelist Agatha Christie. Her experiences in Iraq, and journeys from Britain to the Middle East, formed the background to several of her books, including Murder on the Orient Express, They Came to Baghdad, and Murder in Mesopotamia.

Writing by Dominic Evans; editing by Philippa Fletcher.