The Syrian Observatory for Human Rights said ISIS now controlled half of all territory in Ancient City of Palmyra.

Chilling images emerged today showing decapitated bodies in the streets of Palmyra by ISIS savage terrorist as they stormed the ancient city.

Since ISIS launched its assault on the city on May 13, at least 462 people have been killed in fighting, the Observatory said, including 71 civilians, some of whom were executed by the terrorists.

The Britain-based group said Syrian army troops had pulled back from positions in and around Palmyra making safe guard routs for people to exiting the city.

"ISIS fighters are in all parts of Tadmur, including near the archeological site," Observatory head Rami Abdel Rahman told AFP.

Mohammad Hassan al-Homsi, an activist from Palmyra said: "Syrian army troops withdrew from their positions."

Government war planes responded by carrying out air strikes on ISIS positions in the city.

Since ISIS launched its assault on the city on May 13, at least 462 people have been killed in fighting, the Observatory said, including 71 civilians, some of whom were executed by the terrorists.

The capture of the ancient city, which houses colonnaded streets and ruined temples that date back thousands of years, has raised fears that its treasures could be plundered like archeological sites overrun by the terrorists in Iraq.

Palmyra is also strategically located at the crossroads of key highways leading west to Damascus and Homs, and east to Iraq.

The city is home to a UNESCO World Heritage site, including ancient temples and colonnaded streets, which previously attracted thousands of tourists.