Islamic State jihadists have severely damaged a major Roman monument in the ancient Syrian city of Palmyra, an antiquities official said after visiting the site on Saturday, according to Reuters.

Under heavy Russian air cover, the Syrian army and allied militias drove the jihadist group out of the UNESCO world heritage site on Thursday, two months after it had seized it in a surprise advance.

It was the second time the city had been recaptured from ISIS in the course of Syria's six-year war.

The jihadists retook Palmyra in central Syria on December 11, 2016, just eight months after the army backed by Russia drove them out.

Antiquities official Wael Hafyan told Reuters he had seen serious damage to the Tetrapylon, a square stone platform with matching structures of four columns positioned at each corner. Only four of the 16 columns were still standing.

"The terrorists detonated it... the damage is extensive," he told the news agency. However, he said some of the fallen columns were not destroyed and could be restored using modern conservation techniques.

There was also harm, but less serious, to the facade of a Roman theater, where the damage was to a part that was restored, not original, he said.

ISIS originally overran Palmyra, a UNESCO World Heritage site known as the "Pearl of the Desert", in May of 2015 and proceeded to blow up UNESCO-listed temples and looted relics that dated back thousands of years.

The jihadist group used Palmyra's grand amphitheater for a massacre in which child members of the group killed 25 Syrian soldiers, execution-style, in front of residents.

The jihadists smash ancient structures in Iraq and Syria because they do not conform to their strict interpretation of Islam.