The Islamic State of Iraq and the Levant (ISIL) group is looting ancient sites across Iraq and Syria on an industrial scale and selling on treasures to middlemen to raise cash, Irina Bokova, the head of the UN cultural agency UNESCO, has said.

One-fifth of Iraq's about 10,000 official world-renowned sites were under ISIL control and many have been heavily looted, and it was unclear what was happening in "thousands more" areas, Bokova told a meeting of experts in London on Thursday.

Some sites in Syria had been ransacked so badly they no longer have any value for historians and archaeologists, and UNESCO was also increasingly worried about Libya, she said.

ISIL-controlled territory contains some of the richest archaeological treasures on earth in a region where ancient Assyrian empires built their capitals, Graeco-Roman civilisation flourished, and Muslim and Christian sects co-existed for centuries.

Palmyra statue destroyed

UNESCO's warning came as Syria's antiquities director said ISIL had destroyed a famous statue of a lion outside the museum in the city of Palmyra, known as Tadmur in Arabic.

This deliberate destruction is not only continuing, it is happening on a systematic basis. Irina Bokova, UNESCO

Maamoun Abdelkarim said the statue, known as the Lion of al-Lat, was apparently destroyed last week.

"ISIL members on Saturday destroyed the Lion of al-Lat, which is a unique piece that is three metres tall and weight 15 tonnes," Abdelkarim told the AFP news agency.

"It's the most serious crime they have committed against Palmyra's heritage," he said.

The limestone statue, discovered at the temple of Al-Lat, a pre-Islamic Arabian goddess, dated back to the 1st century BC.

ISIL captured Palmyra, a renowned UNESCO World Heritage site, from government forces in May.

So far, the city's most famous sites have been left intact, but several nearby shrines have been blown up.

Also on Thursday, the group released photos showing its members in Aleppo destroying several statues from Palmyra that were being smuggled through the northern province.

"An ISIL checkpoint in Wilyat [region of] Aleppo arrested a person transporting several statues from Palmyra," the group said in an online statement.

"The guilty party was taken to an Islamic court in the town of Minbej, where it was decided that the trafficker would be punished and the statues destroyed."

The statement included photos showing several carved busts being destroyed with sledgehammers.

Abdelkarim said the busts "appear to be eight statues stolen from the tombs in Palmyra".

'Unprecedented' destruction

In London, Bokova told the Royal United Services Institute that the destruction of artefacts "has reached unprecedented levels in contemporary history.

"This deliberate destruction is not only continuing, it is happening on a systematic basis. The looting of archaeological sites and museums, in Iraq particularly, has reached an industrial scale of destruction."

Such "cultural cleansing" was aimed at destroying humanity's common roots, she said, but was also a source of financing for fighters who she said were charging local farmers to excavate sites and smuggling out artefacts which eventually made their way to private collectors around the world.

"Daesh [ISIL] knows there's a financial upside of this activity and they are trying to gain from it. We know also that parties in the conflict are selling to certain dealers and to private collectors and to market end buyers."

Satellite images helped UNESCO understand what was going on, she said, but in some areas there were just hundreds of holes in the ground from which artefacts were being extracted and it was difficult to understand what was being looted.