LHASA, March 7 (Xinhua) -- Researchers in Tibet Autonomous Region said Thursday that they are cataloguing and making digital copies of antique books at Norbulingka, an ancient palace, amid efforts to better preserve the ancient documents.

The palace's management office and the regional ancient book protection center jointly launched a survey of old documents in the palace last September. So far, the survey has covered nearly 600 books, with over 400 antique books registered as cultural relics under state protection.

Penpa Tsering, deputy head of the regional ancient book protection center, said nine of the books were the only existing copies.

He said the survey team catalogued ancient books, made photocopies and put them into protective cases.

The team will continue to survey over 4,000 ancient books in Norbulingka, located in the regional capital Lhasa.

Built in the mid-18th century, the palace was added to the UNESCO World Heritage List in 2001 as part of the historic ensemble of Potala Palace.

Tibet is home to a massive collection of ancient books, and the region has completed a survey of more than 13,700 ancient books, accounting for over 65 percent of the region's total.