Six ancient sites in Syria, including the city of Aleppo which has suffered considerable damage in the course of the country's ongoing conflict, have been added to the endangered World Heritage list, UNESCO says.

The other sites to be added are the ancient cities of Damascus and Bosra, the oasis of Palmyra, the castles of Crac des Chevaliers and Qal'at Salah El-Din - also known as the Fortress of Saladin - and the ancient villages of northern Syria.

"Due to the armed conflict situation in Syria, the conditions are no longer present to ensure the conservation and protection of the Outstanding Universal Value of the six World Heritage properties," a UNESCO document said.

All six were placed on the list of World Heritage in Danger by the United Nations Educational, Scientific and Cultural Organisation committee at its annual meeting in Phnom Penh.

In preparatory documents for a meeting on Thursday, UNESCO said its information on the scale of the destruction was "partial" and came from unverified sources including social media and a report from the Syrian authorities which it said "does not necessarily reflect the actual situation".

'Caught in the line of fire'

Aleppo's old city, in particular, has "witnessed some of the conflict's most brutal destruction", it said, adding that the old citadel had been "caught in the line of fire".

In April, the minaret of Aleppo's ancient Umayyad mosque - originally built in the 8th century and then rebuilt in the 13th century - was destroyed.

"The immediate, near-term and long-term effect of the crises on the cultural heritage of Aleppo cannot be overstated," UNESCO said.

Clandestine excavations, including looting of ancient tombs and grave sites, have also been reported at several of the sites, it added.

More than 93,000 people, including at least 6,500 children, have been killed since the outbreak of civil war in Syria in March 2011, the UN announced last week in a report that highlighted a surge in the number of deaths each month.