Islamic State militants have destroyed a temple at Syria's ancient ruins of Palmyra, according to activists.

News that the militants blew up the Baalshamin Temple came after extremists beheaded Palmyra scholar Khaled al-Asaad.

Palmyra, one of the Middle East's most spectacular archaeological sites and a Unesco World Heritage site, sits near the modern Syrian city of the same name.

Activists said the militants used explosives to blow up the Baalshamin Temple on its grounds and the blast was so powerful it also damaged some of the Roman columns around it.

The Britain-based Syrian Observatory for Human Rights said the temple was blown up a month ago. Turkey-based activist Osama al-Khatib, who is originally from Palmyra, said the temple was blown up on Sunday. Both said the extremists used a large amount of explosives to destroy it.

The Sunni extremists claim ancient relics promote idolatry and say they are destroying them as part of their purge of paganism. However, they are also believed to sell off looted antiquities.

The Baalshamin Temple is about 500 metres from Palmyra's famous amphitheater where the group killed more than 20 Syrian soldiers after they captured the historic town in May.

The temple dates to the first century and is dedicated to the Phoenician god of storms and fertilising rains.

The head of Unesco, Irina Bokova, said Islamic State extremists in Syria and Iraq are engaged in the "most brutal, systematic" destruction of ancient sites since the Second World War.

Militants have also demolished the St Elian Monastery, which housed a fifth-century tomb and served as a major pilgrimage site. The monastery was in the town of Qaryatain in central Syria.

News of the temple's destruction comes after relatives and witnesses said Khaled al-Asaad, an 81-year-old antiquities scholar who devoted his life to understanding Palmyra, was beheaded by Islamic State militants.