IS killed 81-year-old Khaled Asaad in a square outside the town's museum, said state news agency SANA and the Britain-based Syrian Observatory for Human Rights.

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Maamoun Abdulkarim, the head of the Antiquities and Museums Department in Damascus, told SANA that Mr Asaad's body was later taken to Palmyra's archaeological site and hung from one of the Roman columns.

Palmyra, home to one of the Middle East's most spectacular archaeological sites - a well-preserved, 2,000-year-old Roman-era city at the town's edge - was captured by IS in May.

Mr Abdulkarim said IS tried to get information from Mr Asaad about sites of the town's treasures without success.

The killing of Mr Asaad is the latest atrocity perpetrated by the militant group, which has captured a third of Syria and Iraq.

Since IS overran Palmyra, there have been fears the extremists, who have destroyed famed archaeological sites in Iraq, would demolish its Roman-era city.

The Observatory, which has a network of activists on the ground in Syria, said dozens of people gathered to witness the killing. Mr Asaad had been held by the IS for about a month, it added.

Mr Asaad was "one of the most important pioneers in Syrian archaeology in the 20th century", Mr Abdulkarim said.

SANA said Mr Asaad had been in charge of Palmyra's archaeological site for four decades until 2003, when he retired. After retiring, he worked as an expert with the Antiquities and Museums Department.

Since falling to IS, Palmyra's ancient site has remained intact but the militants destroyed a lion statue in the town dating back to the 2nd century. The statue, discovered in 1975, had stood at the gates of the town museum, and had been placed inside a metal box to protect it from damage.

In early July, IS released a video showing the killing of 20 captured government soldiers in Palmyra's amphitheatre. They were shot dead by young IS members armed with pistols. Hundreds of people were seen watching the killings.