SEOUL (AFP) - South Korean police on Saturday stopped a planned launch of anti-North Korea leaflets across the border, an activist said, as Seoul seeks to open dialogue and defuse soaring tensions with Pyongyang.

The move came as the North issued a fresh threat to bomb any site where leaflets are launched as it accused Seoul of stirring the crisis which has engulfed the Korean peninsula ever since Pyongyang conducted a rocket test last December and a nuclear test in February.

Five activists travelling in a pick-up truck loaded with 100,000 leaflets were surrounded by some 70 policemen near a hill in the northwestern city of Gimpo after which their vehicle was forcibly towed to the local police station.

"The police told us not to send such leaflets in the lead to April 15", the birthday of North Korea's late founder Kim Il Sung which the North celebrates as one of its most important national holidays, activist Park Sang Hak said.

Police have never before blocked such a launch without previously announcing their intentions in the media, Mr Park said, adding that his group have launched leaflets five times over the past two months.

A police official told Yonhap news agency that Gimpo residents living near the border strongly objected to launching leaflets amid mounting tensions over North Korea's nuclear and missile programmes.

Mr Park's group, all defectors from the North, planned to use gas-filled balloons to float the leaflets across the western border.

Pyongyang's official Internet website Uriminzokkiri said such launches were an "intolerable, provocative campaign" aimed at smearing the dignity of its leadership.

"If the puppets let ragtag bunches of misfits carry out such a circus as leaflet launches, a horrible tragedy would take place," it said. "At a moment when even a single rubbish leaflet lands on our side, the origin of the provocation will be blown up."

The statement came after two more groups of anti-Pyongyang activists said they would launch leaflets on Monday to coincide with Kim Il Sung's birthday in order to demand the release of former POWs and fishermen kidnapped by the North.

US Secretary of State John Kerry on Friday demanded North Korea scrap an expected missile test and dial down warlike rhetoric, while backing new peace overtures by Seoul.

Seoul this week urged Pyongyang to return to dialogue. President Park Geun Hye reportedly told ruling party officials on Friday that the South should meet with the North and "listen to what North Korea thinks".