North Korea says it has arrested two South Korean men who it accused of spying for Seoul.
In a statement late on Thursday, state media said the two men, based in the Chinese border city of Dandong, were working to smear North Korea.
South Korea described the allegations as "groundless".
North Korea periodically arrests foreigners and, in the cases of South Korean and US nationals, is accused of using them as bargaining chips.
It also arrests those engaged in missionary work, because religious activity is severely restricted in North Korea.
Sometimes it swiftly deports those held, as in the case of Australian missionary John Short, who was briefly detained in 2014.
But individuals from nations at odds with Pyongyang, or from which it is seeking concessions, are held for longer.
Last year, a South Korean missionary named Kim Jong-uk was given a life sentence after North Korea convicted him of spying and setting up an underground church.
South Korean missionary, identified by the North as Kim Jong-uk, adjusts his glasses during a news conference in Pyongyang on 27 February 2014
Kim Jong-uk is currently serving a life sentence with hard labour in the North
'Isolate and blockade'
Dandong, which lies across the Yalu river from North Korea, serves as a trading hub with the North and a centre from which missionaries and those seeking to help North Korean refugees operate.
In its statement, North Korea said the two men "zealously took part in the anti-DPRK [North Korea] smear campaign of the US imperialists and the puppet group of traitors to isolate and blockade the DPRK in [the] international arena".
The men had gathered information about "party, state and military secrets", the statement said, and one was accused of spreading "religious propaganda" from an underground church in Dandong.
South Korea condemned the move. "It's very regrettable that the North is making such a groundless claim about them," the Unification Ministry said in a statement quoted by Yonhap news agency.
"We strongly call for their quick release and repatriation."
Ties between the two Koreas - which remain technically at war - remain very strained.
Seoul is currently engaged in annual joint military drills with the US, an action that always infuriates Pyongyang.
Trade ties have been cut to a minimum since the deadly sinking of a South Korean warship five years ago. Seoul says Pyongyang torpedoed the ship - North Korea denies this.
Thursday marked five years since the Cheonan went down with the loss of 46 lives. North Korea wants trade links to resume, but the South says an apology for the sinking must come first.
International talks aimed at ending Pyongyang's nuclear ambitions, meanwhile, have been stalled since 2009.
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