BEIRUT: A powerful blast in the ruins of Syria's ancient Palmyra raised fears on Monday that the self-styled Islamic State (IS) group has damaged another of the Middle East's most treasured heritage sites.

Both Syria's antiquities chief and a monitor reported the Sunday blast in the UNESCO World Heritage site, but there was conflicting information on the fate of its famed Temple of Bel.

IS destroyed the smaller Baal Shamin temple at Palmyra last week, confirming the worst fears about their intentions for the site, which they seized from Syrian regime forces in May.

The jihadists have carried out a sustained campaign of destruction against heritage sites in areas under their control in Syria and Iraq, and in mid-August beheaded the 82-year-old former antiquities chief in Palmyra.

The extremist group's harsh interpretation of Islam considers statues and grave markers to be idolatrous, but it has also been accused of destroying heritage sites to loot items for the black market and to gain publicity.

The Syrian Observatory for Human Rights, a Britain-based monitor, said late Sunday that IS fighters had set off explosives inside the 2000-year-old Temple of Bel, at least partially destroying the centrepiece of Palmyra's famed ruins.

Mohammad Hassan al-Homsi, an activist from Palmyra, also reported the partial destruction on Sunday night. “They laid the explosives today, using booby-trapped boxes and barrels that were already prepared by IS,” he said.

“This was the most important temple for tourists and for the people of Palmyra. They used to hold festivals there.“ Homsi, who goes by a pseudonym, said the inner part of the temple was destroyed in the blast.

But Syria's antiquities chief Maamoun Abdulkarim said the explosion did not appear to have damaged the temple significantly.

A view shows the colonnade in the historical city of Palmyra. -Reuters

“The frontal columns and the cella (interior) of the temple do not appear to have been damaged,” Abdulkarim said on Monday.

“According to the information we received from the town, the temple is still standing, but antiquities staff are not able to enter the site to see close up,” he said.

Most important' in Mideast -

There were no immediate images released by IS of the reported destruction.

The reports come a week after IS blew up the Baal Shamin temple, an act the UN's cultural agency UNESCO called a “war crime”.

That destruction was first reported by activists and Abdulkarim and was later shown in photos released by IS. The UN training and research agency UNITAR said that new satellite images of Palmyra confirmed the demolition.

IS captured Palmyra on May 21, sparking international concern about the fate of the heritage site described by UNESCO as of “outstanding universal value”.

Known as the “Pearl of the Desert”, Palmyra, which means City of Palms, lies 210 kilometres (130 miles) northeast of Damascus.

Before the arrival of Christianity in the second century, Palmyra worshipped the Semitic god Bel, along with the sun god Yarhibol and lunar god Aglibol.

Abdulkarim said the Temple of Bel was Palmyra's most important site.

“It is the best example of the combination of Oriental and Greco-Roman art styles,” he said.

“Along with the temple of Baalbek in Lebanon, it is the most important temple in the Middle East. “It is special because its different features are still intact,” he added.

Construction on the temple began in 32 BC and ended in the second century, and it later served as both a church and a mosque.

IS burns four Iraqi Shia fighters alive

The IS group strung up four Iraqi Shia fighters with chains and burned them alive, according to footage posted online, the latest gruesome execution video from the jihadists.

The victims -- identified as fighters in the pro-government Popular Mobilisation forces from southern Iraq -- were suspended from a swingset-like metal structure by chains attached to their hands and feet, then set on fire.

IS, which overran large parts of Iraq last year and still controls much of the country's west, said the murders were in revenge for the alleged burning of four men by pro-government forces.

“Now retribution has come, for today, we will attack them as they attacked us, and punish them as they punished us,” a masked militant says in the video.

The video was not dated and did not give a specific location for where the killings took place, but it did carry a tag indicating that it was produced by the IS media unit responsible for Iraq's Anbar province.

IS has carried out a slew of atrocities in territory it controls in Iraq and Syria, such as mass executions and a campaign of killings, kidnapping and rape targeting minorities.

It has recorded many killings -- including beheadings, shootings, drownings and burnings -- in videos posted online.

Baghdad's forces regained significant ground from the jihadists in two provinces north of the capital with support from a US-led coalition and Iran, but much of western Iraq remains outside government control.