Islamic State (IS) has taken over the ancient city of Palmyra in central Syria days after capturing the Iraqi city of Ramadi. Pro-government forces withdrew Wednesday from the city of Palmyra following a large scale offensive by the militant group, Syrian state media reported.

Palmyra is home of world famous 2,000-year-old ruins, which UNESCO has designated a World heritage site. 

Rami Abdul Rahman, head of the Syrian Observatory for Human Rights, said that at least 100 pro-government fighters have been killed since the militant group began its offensive on Tuesday. Islamic State now controls Palmyra's military air base, prison and intelligence headquarters.

"The Islamic State organisation has now established almost complete control over the area from Palmyra to the Syrian-Iraqi border and onwards to the Syrian-Jordanian frontier," Rahman said, according to The Guardian.

There are fears that the militants will destroy the ruins, as IS had previously demolished several ancient sites that pre-date Islam in Iraq like Nimrud and Hatra, BBC News reported.

"If only five members of IS go into the ancient buildings, they'll destroy everything," Syria's antiquities head Mamoun Abdulkarim told Agence France-Presse.

The head of the United Nations Educational, Scientific and Cultural Organization (UNESCO) has called on all warring factions in Syria to immediately end hostilities within the archaeological site of Palmyra.

The central Syrian city contains the monumental ruins of a great city that was one of the most important cultural centres of the ancient world, UNESCO said.

The Palmyra ruins were one of Syria's foremost tourist attractions prior to the start of the country's civil war in 2011, IANS reported.

"I am deeply concerned by the situation at the site of Palmyra," UNESCO Director-General Irina Bokova said in a press statement. "The fighting is putting at risk one of the most significant sites in the Middle East and its civilian population."