Fighters belonging to the Islamic State of Iraq and the Levant (ISIL) have advanced on Syria's ancient city of Palmyra, one of the Middle East's most famous UNESCO heritage sites, with fierce clashes taking place close to the city's historic citadel.

Photos circulating on social media sites on Saturday appeared to show intense clashes near the 13th century citadel of Fakhr-al-Din al-Ma'ani as ISIL fighters engaged the Syrian military.

Talal Barazi, the governor of central Homs province, where the city is located, said on Friday that the "army has sent reinforcements and it is bombing the [ISIL] positions from the air".

Palmyra, a 2,000-year-old desert oasis, is believed to be home to some 100,000 people, including displaced Syrians who fled there after their home towns were engulfed in violence.

The city is also home to the notorious Tadmor prison, where extensive human rights abuses, torture and summary executions have taken place.

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Irina Bokova, the head of the UN's cultural body UNESCO, called on Syrian troops and ISIL to spare the city, saying it "represents an irreplaceable treasure for the Syrian people and the world".

"Palmyra must be saved," Bokova said at a two-day conference in Cairo on protecting the region's archaeological sites.

Bokova said it was important to work "against extremism, against this strategy of eradicating ... our collective memory."

Nicknamed "the pearl of the desert", UNESCO has described Palmyra a heritage site of "outstanding universal value".

The historical city stood on a caravan route at the crossroads of several civilisations, and its first and second century temples and colonnaded streets house a series of old and beautifully decorated tombs.

However the ancient city, which has previously been a frontline in the four-year-long Syrian conflict, has already been deeply affected by the conflict.

One of its masterpieces, the Temple of Baal, was damaged by artillery exchanges prompting UNESCO to include it in a list of World Heritage sites in danger.

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ISIL, which controls large swaths of territory, has ransacked and demolished several ancient sites, including Muslim shrines, in order to eliminate what it views as heresy, whilst selling artefacts on the black market in order to finance its campaign.

At least 73 soldiers, in addition to 26 civilians, have been killed since the group began its offensive on Tuesday.

Rami Abdel Rahman, the head of the London-based Syrian Observatory for Human Rights which monitors the conflict through a network of local activists, said the families of government officials were among those killed in the village of Amiriyeh, just north of Palmyra.

The Syrian conflict, which began as a peaceful uprising against Assad's rule more than three years ago, has become a bloody and protracted sectarian war killing more than 200,000 people.