North Korea detained two South Korean citizens Friday on espionage charges, in a move that could worsen already-frosty inter-Korean relations.

North Korean authorities announced Kim Guk-gi and Choi Choon-gil were detained for spying on the North. The South denied the accusations and demanded their immediate release.

“We strongly ask that (the North) release our citizens without delay,” a spokesperson for the South’s Unification Ministry said. “We express deep regret over the North’s ridiculous claims.”

But the South’s efforts to free them will be an uphill battle, analysts said, given the North’s record on detaining U.S. and South Korean citizens for months on similar grounds, despite external pressure to release them.

Later Friday, the North refused to accept a letter from Seoul officials requesting their release.

The Unification Ministry also requested for Kim Jong-uk, another South Korean citizen, to be released. He has been detained in North Korea since October 2013.

Kenneth Bae, a Korean-American, was detained in the North for nearly two years from 2012 to late last year. Laura Ling and Euna Lee, two U.S. reporters, were also detained in 2009.

The North asserted that Kim and Choi were found to have relayed information on the North to the South’s National Intelligence Service. Pyongyang authorities added the two had attained information from Chinese residents near the North Korean border in Manchuria.

Kim and Choi admitted their charges at a press conference held in the North. Many in the South believe the conference was staged and the confessions made under duress.

“Their lives probably depended on it,” said Ahn Chan-il, a North Korea expert.

Ahn added that Kim and Choi were likely detained so the North could gain an upper hand over the South in any future negotiations.

“For South Korea to gain (Kim and Choi’s release), it will be forced to appease the North in some way,” he said.

“The North might ask that the South lift sanctions imposed on the North in return for someone’s release,” Ahn added, in reference to the so-called May 24 sanctions the South put on the North in 2010 after Pyongyang torpedoed a southern naval ship, the Cheonan, in March of that year.

There has been debate in Seoul’s legislature in recent weeks over whether it should lift the May 24 sanctions in order to improve worsening ties with the North. But most South Korean conservatives have opposed removing the sanctions, saying the North must first apologize for its sinking of the Cheonan.

North Korea has consistently denied involvement in the sinking, sparking anger with both liberal and conservative South Koreans.

By Jeong Hunny (hj257@heraldcorp.com)