ISIS fighters are reported to have killed at least 400 civilians in the ancient Syrian city of Palmyra, according to Syrian state media reported on Sunday.
Activists have also said many people in the town have been executed, according to Reuters, however the exact number of those killed was unclear.
Following the city's takeover by the extremist group on Thursday, local residents said hundreds of bodies could be found lying around the city's streets.
Militants from ISIS entered the ancient Palmyria ruins on Thursday and were said to be in complete control of the city, according to the London-based monitoring group the Syrian Observatory for Human Rights.
Speaking to Reuters by telephone, the head of the monitor, Rami Abdulrahman, said that the group had stormed and taken control of the city's military air base and prison on Wednesday.
Abdulrahman, who bases his reports on information given by an established network of sources on the ground, added that there was no evidence of any destruction of ruins as of yet.
The city is located in central Syria, in a strategically important area between the capital Damascus and Deir al-Zour in the east. It is also located near gas fields, the BBC said.
Syrian state media reportedly said that forces loyal to President Bashar al-Assad had pulled out of Tadmur — the modern settlement adjacent to the ruins — but claimed troops had done so after "assuring the evacuation" of most inhabitants.
The city is believed to have had a population of 70,000, which is likely to have swollen due to an influx of internally displaced people fleeing other parts of the country, according to the BBC.
Syria's head of antiquities, Maamoun Abdul Karim, described the fight against ISIS in Palmyra as "the entire world's battle," according to the BBC.
Palmyra is a UNESCO World Heritage site dating back to the first and second centuries, and is one of the most important cultural sites from the ancient world. Following ISIS' advance, UNESCO Director-General Irina Bokova said in a statement Wednesday:
"I am deeply concerned by the situation at the site of Palmyra. The fighting is putting at risk one of the most significant sites in the Middle East, and its civilian population.
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. Finally, it is imperative that all parties respect international obligations to protect cultural heritage during conflict, by avoiding direct targeting, as well as use for military purposes."