The Islamic State (Isis) militants have rigged the entire ancient city of Palmyra with explosives and landmines, a UK-based Syria monitoring group said on Sunday.

Palmyra, which is a UNESCO World Heritage Centre, was captured by the radical Islamist terrorist group - Isis in May, after Syrian forces fled the battle.

According to the Syrian Observatory for Human Rights (SOHR), the Isis has planted landmines all over the ancient ruins. "They have planted it yesterday (20 June). 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.

Since May, there have been fears that the Isis may destroy the ancient oasis city of Palmyra, also referred to as Tadmur in Syria.

The SOHR noted that it was not clear yet whether the bombs were planted in the city to destroy the 2000-year-old historical site or it was an attempt to discourage Syrian forces from entering the city.

The Isis took over the city with 50,000 people on 14 May. Even though it had promised the local residents that they would not destroy the monumental ruins of the great city, on 28 May The Daily Mail reported that a famous statue of a lion in the captured city of Palmyra was broken by the Isis.

Earlier this month, the Sunni radical group had released a photo report that shows its fighters destroying dozens of tombs and headstones in the historical city of Palmyra.

There is much concern being raised for Palmyra as the Isis considers all such historical monuments as idolatry, or shirk and thinks destroying such "blasphemous" objects is a blessed act.

In March, the Isis bulldozed Nimrud, another UNESCO site considered a priceless jewel in Iraq's archaeological heritage.

The jihadist group even posted a video online, showing its fighters destroying Nimrud's ancient ruins using barrel bombs, bulldozers and jackhammers.