Tourists walk in the historical city of Palmyra Thomson Reuters

BEIRUT (Reuters) - Islamic State has planted mines and bombs in the ancient part of the central Syrian city of Palmyra, home to Roman-era ruins, a group monitoring the war said on Sunday.

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 theater, we still do not know the real reason," Rami Abdulrahman, the head of the Observatory, told Reuters.

Maamoun Abdulkarim, Syria's head of antiquities, told Reuters that the reports of Islamic State planting bombs in Palmyra "seems true".

"The city is a hostage in their hands, the situation is dangerous."

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.

(Additional reporting Kinda Makieh in Damascus; Editing by Hugh Lawson and Elaine Hardcastle)