Satellite images have confirmed that ISIS has destroyed the Temple of Bel, which was one of the best preserved Roman-era sites in the Syrian city of Palmyra, a United  Nations agency said.

A comparison of before and after images shows the damage to the temple at the UNESCO World Heritage site, the Geneva-based United Nations Institute for Training and Research (UNITAR) said.

"We can confirm the destruction of the main building of the Temple of Bel as well as a row of columns in its immediate vicinity," the agency said in an email.

This came a week after the militants blew up another smaller site in the central desert city.

Palmyra was one of the most important cultural centers of the ancient world, according to cultural agency UNESCO, which has described it as the crossroads of several civilizations. Its main colonnaded street is 1 kilometer (0.6 miles) long and forms the monumental axis of the city.

Information on events inside Palmyra has been patchy since the militants seized it from government forces in May. Activists say the fighters are closely monitoring communications inside the city.

Islamic State has declared a caliphate in territory it holds across Syria and Iraq and has destroyed other monuments it says are pagan and sacrilegious.

UNESCO has called such acts war crimes and says Islamic State seeks to wipe out evidence of Syria's diverse heritage.

Before the capture of the city, Syrian officials said they had moved hundreds of ancient statues to safe locations.

Last month Islamic State beheaded the guardian of Palmyra's ruins, Khaled al-Asaad, an 82-year-old Syrian archaeologist who had looked after the site for more than four decades, according to Syria's antiquities chief.