Speculation continues about the health of Kim Jong-il
North Korea has accused the South of sending an agent to try to assassinate its reclusive leader, Kim Jong-il.
It said the man crossed the border earlier this year and had been planning to use poison to kill Mr Kim.
The man, who is now under arrest, was described by Pyongyang as a North Korean citizen who had received training in the South.
South Korea's main intelligence agency has denied any involvement in the alleged plot.
The North's claim comes at a time of worsening relations between the two Koreas, as well as continued speculation about the health of Mr Kim.
North Korea made the statement via its official news agency, KNCA, late on Thursday.
It said that a man had recently been arrested for trying to conduct a "terrorist mission" that would "do harm to the top leadership".
The man, named as Ri, was under orders from South Korea's intelligence agency, the statement claimed.
"The organisation sent him speech and acoustic sensing and pursuit devices for tracking the movement of the top leader and even violent poison," the statement said.
Although it did not mention Mr Kim as the specific target, the word "leadership" generally refers to him and his top aides.
The North's statement also said unspecified foreign agents had been arrested for trying to gather soil, water and leaves to gain information on Pyongyang's nuclear programme.
South Korea's main spy agency has denied any involvement in any of the allegations.
"This has nothing to do with us," an agency official told reporters.
Mr Kim, who is 66 and reportedly suffering from heart disease and diabetes, has not been seen at key events in recent months.
However, KCNA has been publishing undated photos of him in the past few weeks.
Since Mr Lee came to power in Seoul, relations have deteriorated
Mr Kim's reported illness comes as international negotiations continue over North Korea's nuclear programme.
North Korea said last year it would give up its nuclear programme in return for aid and diplomatic concessions, but progress towards dismantling its Yongbyon nuclear reactor has been patchy.
Since the election in South Korea of President Lee Myung-bak nearly a year ago, relations between the two neighbours have nosedived.
The North cited the Mr Lee's government's "anti-North Korean moves" as its reason for speaking out in its Thursday statement, saying relations have reached "an extremely reckless and dangerous phase".
Mr Lee says he is reviewing a raft of cross-border projects agreed in historic summits in 2000 and 2007, and has linked aid to progress on the issue of North Korea's nuclear disarmament.
On 1 December, North Korea began enforcing stricter border controls with the South, because of what it called "relentless confrontation" from Seoul.
It is impossible to verify the North's claims, but the two nations are known to actively spy on each other, and there have been assassination attempts by both sides in the past.