DAMASCUS/BEIRUT (Reuters) - Islamic State militants have destroyed one of the most famous monuments in the ancient city of Palmyra, the Tetrapylon, and the facade of its Roman Theatre, Syrian antiquities chief Maamoun Abdulkarim told Reuters on Friday.

The Syrian government lost control of Palmyra to Islamic State in December, the second time the jihadist group had overrun the UNESCO world heritage site in the six-year-long Syrian conflict.

UNESCO Director General Irina Bokova said in a statement that the destruction constituted “a new war crime and an immense loss for the Syrian people and for humanity”.

The Tetrapylon, marking a slight bend along Palmyra’s grand colonnade, comprises a square stone platform with matching structures of four columns positioned at each of its corners.

Satellite imagery sent by Abdulkarim to Reuters showed it largely destroyed, with only four of 16 columns still standing and the stone platform apparently covered in rubble.

The imagery also showed extensive damage at the Roman Theatre, with several towering stone structures destroyed on the stage. Just last May, a famous Russian orchestra performed at the theater after Palmyra was first won back from Islamic State.

Abdulkarim said if Islamic State remained in control of Palmyra “it means more destruction”. He said the destruction took place sometime between Dec. 26 and Jan. 10, according to the satellite imagery of the site.

Islamic State had previously captured Palmyra in 2015. It held the city for 10 months until Syrian government forces backed by allied militia and Russian air power managed to drive them out last March.

During its previous spell in control of Palmyra, Islamic State destroyed other monuments there, including its 1,800-year-old monumental arch. Palmyra, known in Arabic as Tadmur, stood at the crossroads of the ancient world.

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Islamic State put 12 people to death in Palmyra earlier this week, some of them execution-style in the Roman Theatre.

Russia marked the capture of Palmyra from Islamic State by sending the Mariinsky Theatre to perform a surprise concert, highlighting the Kremlin’s role in winning back the city.

The concert, held just over a month after Russian air strikes helped push Islamic State militants out of Palmyra, saw Valery Gergiev, a close associate of President Vladimir Putin, conduct the Mariinsky orchestra.

Islamic State swept into Palmyra again in December when the Syrian army and its allies were focused on dealing a final blow to rebels in the city of Aleppo. Eastern Aleppo fell to the government later that month.