North Korea Tuesday sentenced two South Koreans detained there to hard labor for life for spying for the South's intelligence agency, the North's media said, in a move seen as dampening the strained inter-Korean ties.

North Korea's Korean Central Broadcasting Station reported that it handed down life sentences to Kim Kuk-gi and Choe Chun-gil on charges of espionage for the South's state spy agency.

"The North's supreme court held a court session for the two South Koreans who were arrested for suspected spying for the United States and the South," it said. "Kim and Choe were sentenced to hard labor for life on charges of spying."

The two are among four South Koreans detained in North Korea, including missionary Kim Jung-wook and Joo Won-moon, a 21-year-old South Korean student with a U.S. green card.

The North's announcement came as the U.N. High Commissioner for Human Rights opened its office in Seoul on Tuesday, which will be tasked with monitoring the human rights situation in the North. (Yonhap)