South Korea's intelligence agents have arrested a North Korean defector on suspicion he was plotting to kill a fellow defector and vocal anti-Pyongyang activist.

A spokesman for the National Intelligence Service says the defector, identified only as Ahn, was arrested earlier this month at a Seoul subway station where he was supposed to meet Park Sang-hak, who leads a group that regularly sends balloons over the border carrying messages critical of the regime.

Park told reporters Friday that Ahn contacted him and offered to arrange a meeting with a Japanese man willing to support his leafleting campaign. But Park says he was alerted by the spy agency about the possible attack.

Yonhap news agency says Ahn is a former member of a North Korean special military unit who defected in the late 1990s, but disappeared for several years. Yonhap says Ahn was carrying poison needles when he was arrested.

The NIS spokesman would not confirm the report about the poison needles.

A North Korean spy was sentenced to 10 years in prison back in January for plotting to kill Hwang Jang-Yop, a former high-ranking North Korean official who defected to the South in 1997 and became a highly vocal critic of Pyongyang. Two other spies received similar sentences last year for plotting to kill Hwang.

Hwang died of natural causes last October.