It was not immediately possible to verify the account, but it was consistent with reports by activists that the Islamist fighters had carried out executions since capturing the city from government troops.
The militants seized the city of 50,000 people, site of some of the world’s most extensive and best preserved ancient Roman ruins, on Wednesday, days after also capturing the city of Ramadi in neighboring Iraq.
The two near simultaneous victories were Islamic State’s biggest successes since a US-led coalition began an air war against the fighters last year, and have forced an examination of whether the strategy is working.
The Sunni Muslim militants have proclaimed a caliphate to rule over all Muslims from territory they hold in both Syria and Iraq. They have a history of carrying out mass killings in towns and cities they capture, and of destroying ancient monuments which they consider evidence of paganism.
“The terrorists have killed more than 400 people… and mutilated their bodies, under the pretext that they cooperated with the government and did not follow orders,” Syria’s state news agency said, citing residents inside the city.
It added that dozens of those killed were state employees, including the head of nursing department at the hospital and all her family members.
Islamic State supporters have posted videos on the Internet they say show fighters going room to room in government buildings searching for government troops and pulling down pictures of President Bashar Al-Assad and his father.
Activists have said on social media that hundreds of bodies, believed to be government loyalists, were in the streets.
The Syrian Observatory for Human Rights, which monitors violence in the country with a network of sources on the ground, says that some people were beheaded in the town since it fell but has not given an estimate for the death toll among civilians.
It says at least 300 soldiers were killed in the days of fighting before the city was captured.
“A bigger number of troops have disappeared and it is not clear where they are,” Rami Abdulrahman from the Observatory told Reuters.