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.

Islamic State control the Palmyra site after capturing it in December for a second time from Syrian government forces.

Government forces and their militia allies, backed by Russian air power, had taken back the city from Islamic State last March after first losing it in 2015.

Syrian activists say the Islamic State group has killed 12 people it held captive in the ancient city of Palmyra by shooting and beheading them.

The Britain-based Syrian Observatory for Human Rights says four of the victims, teachers and government employees, were beheaded. The Observatory and the activist Palmyra Monitor network say four opposition fighter and four pro-government troops were first shot, then beheaded. The activists say the killings took place on Wednesday.

Some of the killings were carried out in the 2nd century Roman amphitheater, while others were said to have taken place in the courtyard of the Palmyra museum or in a former Russian base in Palmyra.

IS militants recaptured Palmyra in December, nine months after they were expelled from the city in a Russian-backed Syrian government offensive.