SEOUL (AFP) ) A South Korean court jailed an elderly left-wing activist for four years on Friday for making an unauthorised trip across the border to North Korea.

Ro Su Hui, 69, was convicted of violating a strict security law that penalises pro-Pyongyang activity and bans citizens from going to the North without prior permission.

Another activist, Won Jin Wook, received a three-year prison sentence for communicating with North Korean officials to arrange Ro's trip.

Ro entered the North via China in March last year for a memorial service marking the 100th day since the death of long-time ruler Kim Jong Il.

He praised new ruler Kim Jong Un during his time in Pyongyang and was arrested in July when he returned home.

"Stern punishment is inevitable because he made a secret visit to North Korea without permission and was involved in (pro-North Korean) activities, even if he knew that his activities would be used for propaganda," the court said in its ruling.

Seoul did not send any official representatives to Kim Jong Il's funeral and approved trips only by two private delegations.