The Syrian Observatory for Human Rights, which relies on a network of sources on the ground, said explosives were laid at the site of the Roman ruins at the town, also known as Tadmur, in central Homs province on Saturday.

'But it is not known if the purpose is to blow up the ruins or to prevent regime forces from advancing into the town,' said Observatory director Rami Abdel Rahman.

He said regime forces had launched heavy air strikes against the residential part of Palmyra in the past three days, killing at least 11 people.

'The regime forces are to the west outside the city, and in recent days they have brought in reinforcements suggesting they may be planning an operation to retake Palmyra,' he added.

'They have planted it yesterday. They also planted some around the Roman theatre, we still do not know the real reason.'

Maamoun Abdulkarim, Syria's head of antiquities, said reports of Islamic State planting bombs in Palmyra 'seems true'.

'The city is a hostage in their hands, the situation is dangerous', he said. 

Islamic State group fighters captured the city of 50,000 people, which is famed for its extensive and well-preserved Greco-Roman ruins, on May 21.

The city's fall prompted fears the extremist group would seek to destroy the UNESCO World Heritage listed ruins as they have done with heritage sites elsewhere in Syria and Iraq.

But so far there have been no reports of IS damage to the ruins or antiquities in Palmyra, though the group did reportedly enter the city's museum, which was largely emptied of its collection before the jihadist group arrived.

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. 

(dailymail.co.uk)