BEIRUT, May 24 (Reuters) - Islamic State in Iraq and Syria (ISIS) fighters have killed at least 400 people in Syria's ancient city of Palmyra, most of them women and children, since capturing the city four days ago, Syrian state television said on Sunday.

It said it was quoting residents inside the city, which is known as Tadmur in Arabic and is home to renowned Roman-era ruins including well-preserved temples, colonnades and a theatre. 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 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.

ISIS 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," Mr Rami Abdulrahman from the Observatory told Reuters.

Opposition activists have said on social media that hundreds of bodies were in the streets of the city after it was seized by the ultra-hardline group on Wednesday. They said they were believed to be government loyalists.

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 neighbouring Iraq.

The two near-simultaneous victories were ISIS' biggest successes since a United States-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.

Meanwhile, Syrian army air strikes killed at least 300 insurgents and wounded hundreds more during a military operation to free besieged troops at a hospital in north-western Syria, state television said on Sunday.

Dozens of Syrian troops, who had been holed up since April in the Jisr al-Shughour hospital in Idlib province, were freed on Friday.