The Syrian Observatory for Human Rights said it was not immediately clear whether the group was preparing to destroy the ancient ruins or planted the mines to deter government forces from advancing towards the city, also known as Tadmur.

“They have 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.

ISIS 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.