South Korean officials have arrested a North Korean defector on suspicion of plotting to kill high-profile activist Park Sang-hak, reports from Seoul say.
Mr Park is an anti-Pyongyang activist involved in sending propaganda leaflets to the North.
Named only as An, the arrested man is reported to be a former commando in his 40s who defected to the South in the late 1990s.
Reports said he had a poison-tipped needle on him when he was arrested.
Mr Park, another defector from the North, leads a group that flies balloons across the border carrying leaflets criticising the North Korean leadership.
He told AFP news agency that Mr An had asked to meet him on 3 September, but he was warned not to go by intelligence officials.
"An told me by phone that he was to be accompanied by a visitor from Japan who wants to help our efforts. But then I was told by the NIS (National Intelligence Service) not to go to the meeting due to the risk of assassination," Mr Park told AFP.
Intelligence officials have not commented formally on the case, but an official who asked not to be named said that Mr An was in possession of a poison-tipped needle at the time of his arrest.
Ties between the two Koreas - who remain technically at war - have been severely strained in recent months.
South Korea blames North Korea for sinking one of its warships in March 2010 with the loss of 46 lives. North Korea also shelled a Southern border island in November, killing four people.
And North Korea has previously used agents to try to kill targets in the South.
Last year a South Korean court jailed two North Koreans for plotting to assassinate the most senior official ever to defect from North Korea, Hwang Jang-yop.
The two men had admitted pretending to be defectors with a secret mission to kill Mr Hwang. North Korea denied the existence of any such plot.