The taking of the historic city of Palmyra by Islamic State represents "the fall of a civilisation", according to Syria's antiquities chief Maamoun Abdulkarim. Speaking to Reuters today, he said: "Human, civilized society has lost the battle against barbarism. I have lost all hope."

He had earlier said that "If IS enters Palmyra, it will spell its destruction," and if the ancient city falls, "it will be an international catastrophe."

"It will be a repetition of the barbarism and savagery which we saw in Nimrud, Hatra and Mosul," he added.

Irina Bokova, Director-General of UNESCO, said that she was "deeply concerned" by the situation. "The fighting is putting at risk one of the most significant sites in the Middle East, and its civilian population," she said. "I reiterate my appeal for an immediate cessation of hostilities at the site. I further call on the international community to do everything in its power to protect the affected civilian population and safeguard the unique cultural heritage of Palmyra."

The capture of the World Heritage Site by Islamic State is a devastating blow to the region's cultural history. While many of Palmyra's statues have been removed for safety, the large structures and monuments cannot be moved and are vulnerable to destruction of the kind perpetrated in Mosul and Khorsabad, where militants destroyed millenia-old friezes with hammers and pneumatic drills. A tweet from Syrian Activists claimed that bulldozers were already on their way to the site.

Most of the site dates back to the 1st and 2nd centuries when the area was under Roman rule. It is in a strategically significant position which makes it a valuable prize.

Islamic State forces entered the ruins after taking complete control over the area, though so far there have been no reports of the destruction of monuments. Islamic State now controls more than 50 per cent of Syrian territory.

It controls the central city's military air base, prison and intelligence headquarters, having stormed it on Wednesday, said Rami Abdulrahman, head of the Syrian Observatory for Human Rights.

Clashes since Wednesday had killed at least 100 pro-government fighters, he said.

According to Syrian state media, pro-government forces were pulled out of Tadmur, the modern settlement on Palmyra, after "assuring the evacuation" of most of its inhabitants, though this statement has been contested by observers on the ground. Its population is normally around 70,000, but has been swollen by refugees.

The capture of Palmyra and the continued advance of Islamic State represents a further blow for the Assad regime and for the Western-backed regime in Iraq, which has also suffered reverses with the loss of Ramadi.

Additional reporting by Reuters.