The ruins of a 2,000-year-old walled city have been found in a reservoir on China's northeastern border with North Korea, China's official news agency reported Wednesday.

The mud-covered ruins were exposed when the water level in the Yunfeng Reservoir was lowered for repairs, Xinhua News Agency said, citing government officials. The reservoir is on a tributary of the Yalu River, which forms the border of the two countries.

The report said the ruins, near the present-day city of Ji'an, are believed to date to the time of China's Han Dynasty, which existed from 202 B.C. to A.D. 220. But Korea's Koguryo kingdom ruled the area at that time, and Xinhua said the city included tombs of Koguryo design.

Another burial area found some 12 miles away on the reservoir floor has 2,360 tombs also believed to date from Koguryo, Xinhua said.

The Koguryo kings reigned from 37 B.C. to A.D 668 over the Korean Peninsula and northeastern China. The era is regarded as one of the high points of Korean cultural and political power.

China angered Koreans in 2004 when it issued a document describing Koguryo as part of Chinese history. Beijing and Seoul agreed to settle the dispute through academic discussions.

The ruined city's wall is five feet high and 13 feet thick and encloses an area 600 feet by 700 feet, Xinhua said. It is surrounded by a moat.

The dam and reservoir were built in the 1950s, Xinhua said.

Zhang Fuyou, chairman of the Mount Changbai Cultural Society of Jilin Province, said further excavation would required to confirm when the city was built, Xinhua reported.