Eye-witnesses have reported the streets are strewn with bodies on the same day photographs of captured Syrian soldiers have emerged.

It follows the killing of nearly 300 pro-government troops two days after they captured the city, now symbolized by a black ISIS flag flying above an ancient citadel.

Release pictures by ISIS Claiming caputures of syrian soldiers in Palmyra

ISIS has also strengthened its position in neighboring Iraq with terrorist fighters killing more than 500 people when they captured the city of Ramadi, last week, Daily Mail reports.

Syrian state television announced the most recent massacre, quoting residents inside the city, which is known as Tadmur in Arabic.

'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.

Governor Talal Barazi of the central province of Homs, which includes Palmyra, said that ISIS members have 'committed mass massacres in the city of Palmyra' since its capture.

He said ISIS fighters took civilians, including women, to unknown destinations.

But he said retaliation was imminent, adding: 'there is planning, God willing, for a military action in the surroundings of Palmyra'.