DAMASCUS, May 30 -- The Islamic State (IS) militants blew up a military prison in Syria's ancient city of Palmyra on Saturday, according to jihadist websites and opposition activists.

The IS militants rigged the Palmyra military prison and its vicinity with explosive devices and detonated them, destroying large parts of the facility, which was overran after the militant group stormed Palmyra on May 20, according to the Syrian Observatory for Human Rights, a UK-based watchdog group.

Meanwhile, jihadist websites linked to the IS group posted online photos, purporting to show the blast and its aftermath.

Activists said the Syrian troops evacuated all inmates from the prison ahead of the IS attack on Palmyra.

IS has recently released a video, showing its fighters inspecting the prison, which was totally empty, save for some belongings of inmates and unfinished meals, in what appeared to be a quick evacuation ahead of the IS attack.

On May 20, the IS militants took full control of the ancient city of Palmyra in central Syria.

Both the residential parts and the historic parts, which host 2,000-year-old monuments and temples, of Palmyra have fallen to the IS group, according to the London-based monitor group.

An airbase, a central prison and the intelligence headquarters are all in IS hands now, said the monitor group.

The IS offensive against Palmyra started on May 13. The group has since captured the towns of Sukhneh and Amiriyeh and the al-Hail and Arak oilfields.

Mamoun Abdulkarim, general director of Syrian antiquities and museums, told Xinhua that government forces managed to transport all movable ancient items from Palmyra to safe areas.

Abdulkarim expressed grave concern over the fate of the old temples and monuments that cannot be moved.

Palmyra, a UNESCO World Heritage site, contains monumental ruins of a city that was one of the most important cultural centers in ancient times.