Damascus, May 21 (IANS): Islamic State (IS) militants have gained complete control of Syria's ancient city of Palmyra, a Unesco world heritage site, official reports said here on Thursday.

The Syrian Observatory for Human Rights (SOHR) said that an air base, a central prison and the intelligence headquarters are all under the control of the IS, Xinhua news agency reported.

Over 100 pro-government troops were killed in overnight clashes around Palmyra, the SOHR said.

Rising out of the desert, Palmyra contains the monumental ruins of a great city that was one of the most important cultural centres of the ancient world, according to Unesco.

The site, most of which dates back to the first and second century A.D. when the region was under Roman rule, is dominated by a grand, colonnaded street.

Authorities managed to remove around 100 statues before IS fighters reached the Roman-era archaeological site on the city's southwestern edge.

Syrian state television reported that army troops and pro-government militias withdrew after evacuating the remaining civilian residents from the city.

The IS offensive against Palmyra started on May 13. The group has since captured the towns of Sukhneh and Amiriyeh and the al-Hail and Arak oil fields.

However, the SOHR said there are no reports yet of any destruction of artefacts.