Newly discovered ruins in Heilongjiang Province are challenging previous assumptions that there was no organized cultivation of China's far northeastern regions.

"The discovery may change the way we look at the early civilization in the northeast region," said Yang Lin, an archeologist from the China History Museum.

The newly found ruins, located in a river basin some 200 km from the Sino-Russian border, include an ancient city covering an area of 1.2 million sq m with some 370 residential sites and more than 100 castles and garrisons. Researchers believe the relics date back 1,700 to 2,200 years.

Xu Yongjie, deputy director of Heilongjiang Provincial Archaeological Research Institute, said that a castle site in the ancient city has watchtowers, moats and crenellations.

Archaeologists say that the number of residential sites suggests that at least tens of thousands people lived in the area.

"We have not found satisfactory explanation why such a developed agricultural society ever appeared here," said Xu Yongjie. Yang believes that people who inhabited the region were born there, but further research is needed before it is known whether these people had contact with Kokuli and Xianbei cultures.

"We will use DNA analysis if any remains of the dead are found, " said Xu.

The plain is now one of six key commercial grain bases in China.