Satellite images have confirmed the destruction of the main building of a Roman-era temple in the Syrian city of Palmyra, a United Nations agency said, after activists said the hardline Islamic State group had targeted it.

The images show the remains of the Temple of Bel at the Unesco World Heritage site, the Geneva-based United Nations Institute for Training and Research (Unitar) said on Twitter.

"Satellite analysis confirms Temple of Bel ... main building destroyed," it said, posting an aerial picture of the aftermath.

The Syrian Observatory for Human Rights monitoring group and other activists said on Sunday that Islamic State had destroyed part of the temple, one of Palmyra's most important sites. This came a week after the militants blew up another smaller temple in the central desert 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, also known as Tadmur.

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.

The UN's cultural agency Unesco has called such acts war crimes and says Islamic State seeks to wipe out evidence of Syria's diverse heritage.  - Reuters