A satellite image shows the main building of the ancient Temple of Bel in the Syrian city of Palmyra has been destroyed, a UN agency has said.

The image was taken a day after a massive explosion was set off near the 2,000-year-old temple in the city occupied by Islamic State (IS) militants.

Earlier, Maamoun Abdulkarim, head of the antiquities and museums department in Damascus, had said there was conflicting information about the temple, one of the most prominent structures in a sprawling Roman-era complex, because eyewitnesses were unable to approach the site.

But Einar Bjorgo, manager of Geneva-based UN satellite analysts UNOSAT, said a satellite image "unfortunately shows the destruction of the temple's main building as well as a row of columns in its immediate vicinity".

UNOSAT based its findings after comparing the image with one taken on August 27 which showed the main building and columns still intact

Mr Bjorgo said the images were important so the UN cultural agency Unesco could have "objective information" about the situation in Palmyra, which Unesco has designated a world heritage site.

IS, which captured Palmyra from forces loyal to President Bashar Assad in May, destroyed the smaller Temple of Baalshamin in the complex last week and posted images of the destruction days later. Unesco condemned the act as a war crime.

Activists, including a Palmyra resident, said an IS bombing extensively damaged the 2,000-year old temple on Sunday. The resident described a massive explosion, adding that he saw pictures of the damage but could not get near the site.

An IS operative said militants detonated explosives near the temple, without elaborating on how much of it was damaged.

Residents in Palmyra told the official Syrian state news agency IS militants destroyed large parts of the temple and booby-trapped other parts of it, expressing concern they plan to destroy the rest soon.

Amr al-Azm, a former Syrian antiquities official who now is a professor at Shawnee State University in Ohio in the US, said he believed a very large amount of explosives was used and the damage to the Temple of Bel was likely extensive.

"This is the most devastating act yet in my opinion," he said.

Earlier this month, relatives and witnesses said IS militants beheaded Khaled al-Asaad, an 81-year-old antiquities scholar who devoted his life to understanding Palmyra.

IS, which has imposed a violent interpretation of Islamic law across its self-declared "caliphate" straddling Syria and Iraq, says such ancient relics promote idolatry.

The Temple of Bel, dating back to 32 AD, shows a unique merging of ancient Near Eastern and Greco-Roman architecture.