Islamic State militants have blown up the temple of Baal Shamin in Syria's ancient city of Palmyra, the country's antiquities chief Maamoun Abdulkarim has said.

"We have said repeatedly the next phase would be one of terrorising people and when they have time they will begin destroying temples," Abdulkarim told Reuters.

"I am seeing Palmyra being destroyed in front of my eyes. God help us in the days to come."

The destruction of the temple comes barely a week after the militants beheaded 82-year-old scholar Khaled Asaad after detaining and interrogating him for a over a month. After killing him, they hung his body in the main square of Palmyra. Asaad worked for more than 50 years as the head of antiquities in Palmyra.

According to Abdulkarim, Isis militants are also starting to excavate for gold and handing out licences for illicit excavation of the city's treasures.

Isis "placed a large quantity of explosives in the temple of Baal Shamin today and then blew it up causing much damage to the temple. The cella (inner area of the temple) was destroyed and the columns around collapsed," he told AFP.

United Nation's cultural agency, UNESCO chief Irina Bokova told the Associated Press that Isis is engaged in the "most brutal, systematic destruction of ancient sites since World War II."

She said the only recourse is to try and prevent the sale of looted artifacts, thus cutting off a lucrative stream of income for the militants.

AP said on Friday, the militants are believed to have bulldozed the St Elian Monastery which houses a 5th century tomb and served as a major pilgrimage site, according to witnesses.

"We haven't seen something similar since the Second World War. I think this is the biggest attempt, the most brutal systematic destruction of world heritage," Bokova said.