The Islamic State has executed at least 400 people in Syria's ancient city of Palmyra, most of which were women and children.

The kills took place only two days after the terrorist group seized the city, Syrian state television reported. One Palmyra resident told Syria's state news agency that ISIS justified the murders as the victims' punishment for cooperating with the government.

Eyewitnesses reports that the dead bodies were strewn on the streets, and that some were even decapitated, according to Daily Mail. 

Among those killed were state employees, as well as a nurse who some claim also holds American citizenship. Supporters of the Islamic State posted videos showing the militants entering governmental buildings in search of Syrian soldiers. They also pulled down photos of Syrian President Bashar Assad and his father, Hafez Assad.

The Syrian Observatory for Human Rights said the battles that took place before the city's capture resulted in the deaths of at least 300 troops.

"A bigger number of troops have disappeared and it is not clear where they are," Rami Abdulrahman, from the monitoring group, told Reuters.