Seoul (AFP) - South Korean activists launched tens of thousands of anti-Pyongyang leaflets across the border into North Korea on Thursday, denouncing its latest nuclear test and defying threats of retaliation.

The leaflets, criticising leader Kim Jong-Un for putting nuclear weapons before the wellbeing of his people, were launched with helium balloons from the border city of Paju.

The propaganda exercise, organised by North Korean defector-turned-activist Park Sang-Hak, came amid surging military tensions on the divided Korean peninsula following the North's fifth and largest-ever nuclear test last week.

Hours before the balloon launch was scheduled to begin, the North's official KCNA news agency published a commentary describing Park as "human scum without an equal in the world."

KCNA said the balloon launch was a desperate response to the success of last week's test, and a bid by Seoul to "stoke confrontation" over the holiday period.

The launch came in the middle of the three-day Chuseok harvest festival holiday -- celebrated on both sides of the border.

Conservative South Korean activists, including many North Korean defectors, have been carrying out leafleting exercises using giant helium balloons for years -- a practice that infuriates Pyongyang which has threatened military strikes in response.

Park Sang-Hak said strong winds at the border had restricted Thursday's event to the launch of around 150,000 leaflets -- half the planned number.

"We are doing this to inform the 20 million starving people in North Korea of the truth," Park told AFP.

"At this moment, when hundreds of thousands of people are suffering from terrible floods, Kim Jong-Un conducted another nuclear test.

"So, who is calling who 'human scum?'" he said.

North Korean state media has described ongoing floods as the worst to hit the country since World War II.

According to a UN agency report, 138 people have died and 400 are missing after torrential rains caused devastation in the country's far north.