PARIS (AFP) - The destruction of the Unesco World Heritage site of Palmyra would be an "enormous loss to humanity", the head of the organisation warned on Thursday after Islamic State in Iraq and Syria (ISIS) fighters seized the ancient Syrian city.

"Palmyra is an extraordinary World Heritage site in the desert and any destruction to Palmyra (would be) not just a war crime but... an enormous loss to humanity," said Irina Bokova in a video published by the Paris-based group.

She added that she was "extremely worried" about recent events there and reiterated an appeal for an immediate ceasefire and withdrawal of military forces.

"At the end of the day, it's the birthplace of human civilisation. It belongs to the whole of humanity and I think everyone today should be worried about what is happening," added the Unesco chief.

Earlier on Thursday, ISIS militants seized full control of the city, putting the world heritage site and its priceless artefacts at risk of destruction.

The extremists, notorious for demolishing archaeological treasures since declaring a "caliphate" last year straddling Iraq and Syria, fought their way into Palmyra on foot after breaking through in the city's north.

Bokova urged the international community, including the UN Security Council and religious leaders, to launch an appeal to stop the violence.

"It is important because we are speaking about the birth of human civilisation, we are speaking about something that belongs to the whole of humanity."