The Islamic State of Iraq and the Levant (ISIL) group has blown up a 2,000-year-old temple in the UNESCO-listed Syrian city of Palmyra, a rights group and the country's antiquities chief have said.

Maamoun Abdulkarim, Syria's antiquities chief, said the group placed a large quantity of explosives in the temple of Baal Shamin on Sunday and blew it up, causing much damage.

"Our darkest predictions are unfortunately taking place," Abdulkarim told the AFP news agency.

ISIL "carried out executions in the ancient theatre [of Palmyra], they destroyed in July the famous Lion Statue of Athena... and transformed the museum into a prison and a courtroom."

The Syrian Observatory for Human Rights confirmed the temple had been destroyed but said activists had told it the incident happened about a month ago.

Another Turkish-based activist, who is originally from Palmyra, told the AP news agency that sources from the city said the temple was blown up on Sunday.

News has only been trickling from Palmyra - a city described by the United Nations Educational, Scientific and Cultural Organization (UNESCO) as of "outstanding universal value" - since it fell under ISIL control in May.

Baal Shamin was built in 17 AD and was expanded under the reign of Roman emperor Hadrian in 130 AD.

It is one of the most important sites in Palmyra, known as the "Pearl of the Desert", a previously well-preserved archaeological oasis 210km northeast of Damascus.

Palmyra's name first appeared on a tablet in the 19th century BC as a stopping point for caravans travelling on the Silk Road and between the Gulf and the Mediterranean.

ISIL looting heritage sites on 'industrial scale'

On Tuesday, ISIL beheaded Khaled Asaad, a respected 82-year-old archaeologist who worked for 50 years as head of antiquities in Palmyra.

Asaad had been detained and interrogated for a month before he was killed. His family said ISIL mutilated his body after his murder.

In July, the group destroyed a famous statue of a lion outside Palmyra's museum, after taking over and looting the city, known as Tadmur in Arabic.

Most of the pieces inside the museum had been evacuated by antiquities staff before ISIL arrived.

ISIL has previously claimed responsibility for destroying shrines - Christian and Muslim - as well as churches. It has vowed to rid territory it controls in Syria and Iraq of symbols of what it calls idolatry.

The head of UNESCO, Irina Bokova, told the AP on Friday that ISIL in Syria and Iraq was engaged in the "most brutal, systematic" destruction of ancient sites since World War II.

Her warning came hours after the group demolished the Saint Elian Monastery, which housed a fifth-century tomb and served as a major pilgrimage site, in the town of Qaryatain in central Syria.

In May, Bokova called on Syrian troops and ISIL to spare Palmyra, saying it "represents an irreplaceable treasure for the Syrian people and the world".