Damascus: The Islamic State (IS) militants have executed at least 17 people since they took control over the ancient city of Palmyra on Wednesday, a monitor group said on Thursday.

Some of the slain people were government forces along with people who are loyal to the government, according to the Syrian Observatory for Human Rights.

The London-based watchdog group said some of the executed people were beheaded by the terror group, following the large-scale offensive on Palmyra on Wednesday.

Earlier in the day, the Observatory said IS militants have taken full control of the ancient city of Palmyra in central Syria.

Both the residential parts and the historic parts, which host 2,000-year-old monuments and temples, of Palmyra have fallen to the IS group, according to the London-based Observatory.

An airbase, a central prison and the intelligence headquarters are all in IS hands now, said the monitor group, which says it relies on a network of activists on the ground inside Syria.

About 100 government soldiers were killed on Wednesday in the battles in Palmyra, the group reported, saying IS militants now control half of Syria.

Late on Wednesday, Syria's official al-Ekbarieh TV said the Syrian National Defence Forces pulled out of Palmyra due to intense IS assaults after evacuating civilians from the city.

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

Mamoun Abdulkarim, general director of Syrian antiquities and museums, told Xinhua that government forces managed to transport all moveable ancient items from Palmyra to safe areas.

Abdulkarim expressed grave concern over the fate of the old temples and monuments that cannot be moved.

Palmyra, a UNESCO World Heritage site, contains monumental ruins of a city that was one of the most important cultural centres in ancient times.

From the 1st to the 2nd century, the art and architecture of Palmyra, standing at the crossroads of several civilisations, married Graeco-Roman techniques with local traditions and Persian influences, according to the United Nations Educational, Scientific and Cultural Organisation (UNESCO).

Syria has many prehistoric, Greek, Byzantine and Islamic heritage sites.

In addition to Palmyra, the UNESCO has listed several other Syrian sites on its World Heritage List, including the old cities of Damascus and Aleppo, al-Madhiq castle, the Krak des Chevaliers, the ancient city of Bosra, and ancient villages in northern Syria.