Islamic State (IS) militants have planted landmines and explosives around the ancient Palmyra ruins, according to a group that monitors the war in Syria, BBC News reports.

Activists said it was unclear whether IS had laid the bombs to destroy the ruins or make the site secure from Syrian government forces.

IS fighters seized the city, which is one of the most important historic sites in the Middle East, in May.

Government forces are reported to be planning a bid to recapture the site.

The director of the UK-based Syrian Observatory for Human Rights, Rami Abdel Rahman, told reporters that Syrian soldiers outside the city have brought in reinforcements in recent days, "suggesting they may be planning an operation".

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

The group relies on a network of sources inside Syria for its information.

IS took control of the ancient city last month, prompting fears the group would destroy the 2,000-year-old Roman-era ruins.

They have already destroyed ancient sites in Iraq - most recently the ancient city of Nimrud, one of Iraq's greatest archaeological treasures.

But so far there have been no reports of IS damage to the ruins or antiquities in Palmyra, which is known locally as Tadmur.