SEOUL (REUTERS) - North Korea said late on Thursday it had arrested two South Koreans based in the Chinese border city of Dandong, accusing them of spying for South Korea.

The North's official KCNA news agency showed images of two middle-aged men it identified as Kim Kuk Gi and Choe Chun Gil speaking at a news conference in the North Korean capital, Pyongyang.

The two men were South Korean nationals working as spies for Seoul's National Intelligence Service from the Chinese border city of Dandong, it said.

"They zealously took part in the anti-DPRK smear campaign of the US imperialists and the puppet group of traitors to isolate and blockade the DPRK in (the) international arena," the agency said, using North Korea's official DPRK acronym for Democratic People's Republic of Korea.

The media office at South Korea's National Intelligence Service did not answer phone calls late on Thursday seeking comment on the KCNA report.

North Korean state media accused one of the men of running a China-based "underground church" and illegally spreading foreign information on USB sticks and SD cards in the country.

Dandong is home to many ethnic Korean Chinese traders who deal with both North and South Korean businessmen. It is also home to South Korean and western Christian missionaries trying to operate in North Korea.

The article did not say how or where the two men had been arrested.

North Korea has held South Korean missionary Kim Jeong-wook since October 2013 on allegations of espionage, despite pleas from Seoul to release him. He was given a life sentence of hard labour.

Last year, a Canadian couple who worked with North Korean refugees and ran a coffee shop in Dandong were accused by the Chinese government of espionage.

Korean-American missionary Kenneth Bae was released last year after being sentenced by the North Korean government on charges of trying to bring down the state.