South Korean police have raided the office of an activist who said he had floated hundreds of thousands of propaganda leaflets towards North Korea by balloon in defiance of a new law.

Propaganda leaflets have emerged as a new source of animosity between the two countries, with Pyongyang calling it a provocation and threatening to retaliate.

Seoul Metropolitan Police said the raid on the office of Park Sang-hak in the South Korean capital was related to his announcement that his group launched balloons carrying 500,000 leaflets, 5,000 one-dollar notes and 500 anti-Pyongyang booklets across the border last week.

They refused to provide details, including whether they plan to question him, citing an ongoing investigation.

Mr Park, a North Korean defector who rose to fame because of his leaflet campaign, issued a brief statement saying officers had arrived at his office.

He later told reporters: “Even if we get three years in prison or even 30 years in prison… we’ll continue to send anti-North leaflets to let our ragged, starving compatriots in North Korea know the truth.”

He earlier said he would fight the new law banning cross-border leaflets and keep launching them to let the North Korean people learn about the truth of their authoritarian government led by Kim Jong Un.

South Korean officials have not publicly confirmed that Mr Park had sent the leaflets, but Mr Kim’s powerful sister, Kim Yo Jong, said North Korean defectors in the South recently “scattered leaflets against” the North.

She called their action “an intolerable provocation” and said her government would look into corresponding measures.

Her statement caused concern that North Korea might launch some sort of provocation against South Korea.

Last year, Pyongyang blew up an empty inter-Korean liaison office on its territory after Ms Kim reacted angrily to a similar incident in which South Korean civilians sent leaflets into North Korea.

If confirmed, Mr Park would be the first person to face the South Korean law that punishes such acts with up to three years in prison. The law took effect in March.

South Korean officials have said they would handle him in line with the law, but any harsh treatment could deepen criticism that Seoul is sacrificing freedom of speech to improve ties with its rival.

Officials say the law is designed to avoid provoking North Korea unnecessarily and to promote the safety of South Korean residents in border areas.