A South Korean activist said Wednesday he had launched thousands of copies of Hollywood film "The Interview" into North Korea by balloon, ignoring dire threats of reprisals from Pyongyang.

The capital has labelled the Seth Rogen comedy about a fictional CIA plot to assassinate North Korean leader Kim Jong-Un, a "wanton act of terror".

North Korean defector-turned-activist Lee Min-Bok said he had carried out four cross-border balloon launches since January -- the latest one on Saturday.

On each occasion he tied bundles carrying copies of "The Interview" and anti-Pyongyang leaflets to helium balloons, which he then released from the back of a truck.

"I launched thousands of copies and about a million leaflets on Saturday, near the western part of the border," Lee told AFP.

All the launches were carried out at night with little or no advance publicity, given the sensitivity on both sides.

North Korea has long condemned the cross-border launches and demanded that the South Korean authorities step in to prevent them.

Last October North Korea border guards attempted to shoot down some balloons, triggering a brief exchange of heavy machine gun fire between the two sides.

Pyongyang issued some particularly stern warnings against any effort to include copies of "The Interview" in the balloon bundles, saying that any challenge to its "just physical countermeasures" will trigger "merciless retaliatory strikes".

It stands accused by the FBI of being behind a devastating cyber attack last November on Sony Pictures, the studio behind the movie.

While appealing to activists to avoid overly provoking the North, Seoul insists their actions are protected by freedom of expression principles.

Police have intervened to prevent some launches, but only when there is a prospect of North Korean retaliation that might endanger residents living near the balloon launch site.

Lee's launches were done at night in remote locations, and though they were monitored by local police, no move was made to stop him.

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"The police would have no right to stop me from doing this," Lee said.

"I am always being tailed by police," he added.

A CNN camera crew that followed Lee on Saturday filmed him attaching the bundles to the balloons in the middle of the night, before releasing them into the darkness.

The balloons are wholly at the mercy of the prevailing winds, and it is impossible to determine how many will actually come down in North Korea.

Seoul's Unification Ministry, which said it had only become aware of Lee's latest launches in the past couple of days, declined to comment directly on his efforts to send copies of the movie.

"Our stance is that we continue to acknowledge the freedom of individuals to publicise their opinions," a ministry spokesman said.