"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.

Syria has six World Heritage Sites: the ancient cities of Damascus, Bosra and Aleppo, 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.

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

In preparatory documents for the meeting, UNESCO said its information on the scale of the destruction was "partial" and came from sources including social media and a report from Syrian authorities.

Aleppo's old city, in particular, has witnessed some of the conflict's most 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.