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

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 the militants in the course of Syria’s six-year war.

Antiquities official Wael Hafyan said 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 Reuters. 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.