Isis militants have reportedly seized part of the ancient Syrian city of Palmyra, a Unesco World Heritage site.

Why Palmyra is so important:

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A third of Palmyra has now been taken, according to the The Syrian Observatory for Human Rights monitoring group.

It said militants had seized the northern part of the city, which lies on a key strategic east-west route.

But the primary international concern has been for the city's 2,000-year-old ruins. The city, known as "an oasis in the Syrian desert", was once a tourist hotspot before civil war broke out in Syria and contains a famous colonnaded main street and the temple of Baal.