A 2,000-year-old city described by Unesco as "one of the most important cultural centres of the ancient world" is in danger of being destroyed by Islamic State (Isis). Ancient temples, a Roman amphitheatre and a majestic colonnade could be lost forever.

The London-based Syrian Observatory for Human Rights (SOHR) said IS fighters had entered the ancient ruins of the 2,000-year-old world heritage site after seizing the city's military air base, prison and intelligence headquarters.

Palmyra, known as Tadmur in Arabic, is a Unesco World Heritage Site sprawling over a 120-acre site in the desert.

An ancient metropolis and a caravan hub of several civilisations, Palmyra is full of precious sculptures and artefacts. Before Syria's civil war started, tourists flocked to its temple ruins and grand Roman colonnade.

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The ancient city of Palmyra covered an area of about 50 hectares (120 acres)Eustache Diemert
Palmyra Isis Syria
The monumental arch in the eastern section of Palmyra's colonnadeiStock
Palmyra Isis Syria
The Great Colonnade at Palmyra was built during the second and third centuries and stretched for more than a kilometre. It linked the Temple of Baal to the West Gate and the Funerary TempleiStock
Palmyra Isis Syria
The Temple of Baal dates back to about 32AD, and was the main site of worship in PalmyraFlickr/upyernoz
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Temple of BaalshaminiStock
Palmyra Isis Syria
The Roman Theatre at Palmyra dates back to the second centuryiStock
Palmyra Isis Syria
Fakhr-al-Din al-Maani Castle, built around 1230, stands on a hill overlooking the historic siteAFP
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Palmyra Castle is seen behind a row of columnsiStock
Palmyra Isis Syria
Gate of the fortified Temple of BaalCreative Commons
Palmyra Isis Syria
The courtyard of the sanctuary of BaalJoseph Eid/AFP
Palmyra Isis Syria
The Agora of PalmyraiStock

From the first to the second century, the art and architecture of Palmyra merged Greco-Roman techniques with local traditions and Persian influences.

Earlier this year, some of its temples were damaged by shelling during clashes between President Bashar al-Assad's forces and rebels.

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The external courtyard of the sanctuary of BaalJoseph Eid/AFP
Palmyra Isis Syria
This photo, taken in March 2014, shows damage caused by shellingJoseph Eid/AFP

Islamic State, which controls parts of Iraq and Syria, has been filmed destroying pre-Islamic artefacts and monuments. In March it razed areas of the 2,700-year-old city of Khorsabad, famous for its colossal statues of human-headed winged bulls.

It has also attacked sites and destroyed antiquities in the cities of Nineveh, Nimrud and Hatra in Iraq.

Hundreds of statues were moved from the historic city to more secure locations as IS militants approached, the country's antiquities chief said. Maamoun Abdulkarim called on the Syrian army, opposition and international community to save the World Heritage site.

"Hundreds of statues we were worried would be smashed and sold are all now in safe places," he said. "The fear is for the museum and the large monuments that cannot be moved. This is the entire world's battle."