A group of North Korean defectors floated hundreds of thousands of propaganda leaflets into the communist nation Tuesday, denouncing Pyongyang for its deadly sinking of a South Korean warship four years ago.

The event came on the eve of the fourth anniversary of the March 26 sinking that killed 46 South Korean sailors near the western sea border with North Korea.

"Now that four years have passed since the sinking of the Cheonan, the government has started to show less interest, while North Korea still hasn't apologized, so we decided to organize this event," said Park Sang-hak, the leader of Fighters for Free North Korea, a group of defector activists.

The defectors, together with members of the conservative South Korean group, National Action Campaign for Freedom and Democracy in Korea, launched 600,000 leaflets vowing revenge on behalf of the victims and condemning the regime of North Korean leader Kim Jong-un.

The leaflets were tied to 20 large balloons and floated across the border from the South's northwestern city of Paju.

South Korean activists and defectors frequently send hundreds of thousands of anti-North Korea leaflets across the border by balloon, despite the North's repeated threats of retaliation.

The Koreas have remained divided by the border since the 1950-53 Korean War ended in a truce, not a peace treaty. (Yonhap)