South Korea urged North Korea to immediately release two of its citizens detained in the country over alleged espionage charges, the latest in a series of arrests in the North of foreign nationals.

The North's state media said Thursday that the two were detained last year for allegedly collecting confidential information about the North's military, ruling party and other state organizations. It identified the men as Kim Kuk Gi and Choe Chun Gil, saying the two acknowledged during a news conference in Pyongyang that they had attempted to collect information about the North's leadership, and create economic chaos by circulating counterfeit money and spreading "unsound" publications to the public.

On Friday, Lim Byeong Cheol, a spokesman at South Korea's Unification Ministry confirmed that Kim and Choe were South Korean citizens but denied they were engaged in espionage operations. Ministry officials could not explain how the two ended up in the North.

"We strongly demand North Korea to quickly release our citizens Kim Kuk Gi and Choe Chun Gil and repatriate them without hesitation," Lim said.

South Korea's National Intelligence Service, the country's main spy agency, denied the North's accusations of spying.

The North's Korean Central News Agency said Kim was detained in September in Pyongyang and Choe near the border with China in December.

North Korea has occasionally detained South Korean nationals in the past on accusations of spying in what outside experts say are attempts to pressure Seoul or raise North Korean people's hostility toward the South.

Last year, North Korea sentenced a South Korean Baptist missionary to hard labor for life for allegedly spying and trying to set up underground churches in the North.

The latest incident comes amid ongoing regular military drills between South Korea and the U.S., which Pyongyang calls a rehearsal for an invasion. South Korean and U.S. officials say the drills are defensive in nature.

Thursday also marked the fifth anniversary of a sinking of a South Korean warship that Seoul officials blame on a torpedo attack by the North. Pyongyang has denied any involvement in the sinking that killed 46 South Korean sailors.