A cultural heritage official has said the ancient city wall of Pingyao City in north China's Shanxi Province will be repaired before a freeze sets in.

Part of the 630-year-old city wall collapsed Sunday afternoon. The local government said the collapse caused no casualties because no one was around when the 17-meter long, 10-meter tall, three-meter thick section close to the southern city gate suddenly fell apart.

A preliminary investigation suggests the collapse was a result of structural defects in the wall itself, instead of human activities, said Li Shusheng, vice director of the cultural heritage administration of Pingyao.

Li told Xinhua in an exclusive interview that the collapsed section was in itself crisp, basified and more prone to fall compared with other parts of the wall.

Sample soil tests also indicated the soil was too loose to support the wall, and construction workers 600 years ago had obviously failed to tamp it hard enough, Li said.

"We've also found the collapsed wall is thicker on the upper and lower parts, but thinner in the middle, a sign of poor brickwork," he added.

Cultural heritage protection experts from the provincial capital Taiyuan have arrived in Pingyao to discuss the repair work, according to Li. "We'll do all what we can to restore its original look before a freeze sets in."

The existing city wall of Pingyao was rebuilt in 1370 on the basis of an old one. The city wall is around 10 meters tall and has a circumference of nearly 6,200 meters.

Pingyao was known in history as China's financial hub. It has well-preserved streets, stores, temples and residential houses built in the Ming (1368-1644) and Qing (1644-1911) dynasties.

The ancient city was listed as a World Cultural Heritage site in 1997 by the United Nations Educational, Scientific and Cultural Organization.