Six armed North Korean border guards tasked with preventing defections have themselves fled across the frontier into China, sparking a security alert on the Chinese side of the border, a report said.

Daily NK, a Seoul-based online newspaper run by North Korean defectors, said in a report posted late Wednesday that the border patrol agents crossed the Yalu river marking the northwestern border around November 20.

Seoul's intelligence agency is investigating whether the report is correct, its spokesman said.

"Eight crossed the river at night but two of them were shot dead by other (North Korean) border agents and only six managed to run away," Daily NK quoted a source in the Chinese border city of Dandong as saying.

"Two teams of agents on night duty conspired together to escape," it said.

Armed Chinese soldiers and the North's intelligence officials have been sent to Dandong to strengthen security checks at train stations and in areas with a large ethnic Korean population, it added.

The North Koreans crossed into Kuandian Manchu Autonomous County 75 km (47 miles) northeast of Dandong, the paper said.

More than 21,700 North Koreans have fled their impoverished and hunger-stricken homeland since the 1950-1953 war, the vast majority in recent years. But defections by soldiers are rare.

Refugees typically escape on foot to China, hide out and then travel to a third country to seek resettlement in South Korea.

China considers such North Koreans as economic migrants rather than political refugees and repatriates them, despite criticism from rights groups that they face harsh punishment back home.