Police in the South Korean capital Friday raided the office of activist Park Sang Hak, a North Korean defector who has been leading a campaign that has floated thousands of leaflets, USB sticks, brochures and dollar bills across the North Korean border by balloon.

During a recent flare-up in tension between the Koreas, the North's earliest and loudest complaints revolved around the leaflet issue. Seoul then responded, introducing laws designed to stop the practice, but it did little to satisfy Pyongyang. 

This week, at roughly the same time as the North claimed it was sending "millions" of leaflets south, Park Sang Hak responded in kind saying he would be sending his own north, potentially prompting Friday's raids. 

Investigators said they confiscated leaflets and account books and will summon Park soon for an investigation. Park has yet to issue a statement responding to the raid. 

"The search is part of efforts to seize materials so that we could verify if Park's activities are in breach of law," a Seoul police official told South Korea's Yonhap news agency. 

Read more:  What's driving North Korea's aggression toward the South?

South Korean Officials say Park's actions could further raise animosity on the Korean Peninsula and potentially endanger lives of people living near the border with the North.

The North already has used the balloon campaign as perhaps its most common justification for a string of recent blows to inter-Korean ties, including blowing up an empty South Korean liaison office on its territory. 

Pyongyang has also threatened to resume military exercises, reestablish some guard posts within the demilitarized zone and float over its own anti-South leaflets. On Wednesday, however, ahead of the 70th anniversary of the outbreak of the Korean war, Pyongyang said it would postpone taking these steps. 

Read more: Korean War anniversary: Are North and South close to another military confrontation?

'Freedom fighters' 

Park, who leads the activist group Fighters for a Free North Korea, has been floating balloons across the border for years. However, the recent belligerent response from the North put a spotlight on the group's activities and has drawn calls by South Korean officials for investigation. The issue of critical propaganda has been a sensitive one for Pyongyang in the past. 

Both the South and North have repeatedly warned Park against sending the balloons. Despite these warnings, on Monday, the group covertly sent out 20 huge balloons in the middle of the night from the border town of Paju carrying 500,000 leaflets, 2,000 one-dollar bills, and small books. 

South Korean officials said It was unclear whether the balloons made it across and the South Korean Ministry of Unification expressed "deep regret" over the action. 

In Gyeonggi province on the border with the North, authorities have requested a separate investigation into several activist groups, including Park's, over alleged fraud and embezzlement charges related to their donation activities. 

wmr/msh (AP, dpa)