Islamic State militants have planted mines and bombs in the ancient part of the central Syrian city of Palmyra, home to Roman-era ruins, the Syrian Observatory for Human Rights, monitoring the war said on Sunday.

The observatory group said it was not immediately clear whether the militants were preparing to destroy the ancient ruins or planted the mines to deter government forces from advancing towards the city, also known as Tadmur.

“They planted it yesterday. They also planted some around the Roman theatre, we still do not know the real reason,” Rami Abdulrahman, the head of the Observatory, told Reuters.

The ultra-hardline Sunni Muslim group in May seized the city of 50,000 people, site of some of the world’s most extensive and best-preserved ancient Roman ruins.

Islamic State has proclaimed a caliphate to rule over all Muslims from territory it holds in both Syria and Iraq. Its militants have a history of carrying out mass killings in towns and cities they capture and of destroying ancient monuments which they consider evidence of paganism.

*Reuters/NAN