PAJU, SOUTH KOREA - The son of a hijacking victim and his supporters on Sunday once more demanded North Korea return his father, five decades after Pyongyang diverted a plane carrying 50 South Koreans on board.

Hwang Won was on a domestic Korean Air flight from Gangneung to Seoul's Gimpo airport on December 11, 1969, only for the aircraft to be hijacked by a North Korean spy 10 minutes after takeoff and made to land in Pyongyang.

Two months later, 39 passengers were repatriated, but the North never returned 11 people including Hwang -- whose son, who was two at the time of the hijacking, has continued to fight for his release.

Returning passengers said Hwang Won had been dragged away after resisting indoctrination efforts and questioning North Korea's ideology.

Hwang In-cheol, now in his 50s, was joined Sunday by dozens of supporters at Imjingak, just south of the Demilitarized Zone (DMZ) dividing the two Koreas, to commemorate the 50th anniversary of the hijacking next Wednesday.

"World leaders won't do anything about this until enough of us speak up," Hwang said in a petition calling for the release of his father.

"North Korea is the worst country in the world, and will remain so until humanity forces it to change," he said.

North Korean leader Kim Jong Un has engaged in a flurry of diplomacy since last year, including summits with US President Donald Trump and the South's President Moon Jae-in, but activists say the issue of human rights has largely been off the table.

With temperatures dipping below zero in the bitter weather, activists waved posters reading, "We stand with Hwang In-cheol", and flew paper airplanes with messages to his father that said: "See you this Christmas!"

Hwang wrote on a ribbon "North Korea, send my father home" and tied it on a wall of other ribbons on a barbed wire fence. -AFP