South Korea's police chief has ordered a swift and thorough probe into a North Korean defector group's alleged sending of anti-Pyongyang leaflets into the North last week, police officials said Monday.

Commissioner General Kim Chang-yong gave the order to the Seoul Metropolitan Police Agency on Sunday, hours after the North Korean regime threatened "corresponding action" in response to the "intolerable provocation."

"I ask you to strictly handle the sending of anti-North Korea leaflets through a swift and thorough investigation," Kim was quoted as saying.

Park Sang-hak, a vocal North Korean defector heading Fighters for a Free North Korea, claimed Friday that his group had sent a total of 10 balloons carrying around 500,000 leaflets, 500 booklets and 5,000 $1 bills on two occasions between April 25-29. Park released a video but stopped short of disclosing the exact launch sites, saying they were sent from Gyeonggi and Gangwon provinces.

If confirmed, it would mark the first time anti-Pyongyang leaflets were sent across the border following the institution of a government ban on such activity in March.

Under the revised Development of Inter-Korean Relations Act, violators are subject to a maximum prison term of three years or a fine of up to 30 million won ($26,870).

Police officials said the commissioner general reprimanded the police force for failing to take appropriate action against the leafleting at a sensitive time ahead of President Moon Jae-in's May 21 summit with US President Joe Biden in Washington.

Last year, the North blew up an inter-Korean liaison office in its border city of Kaesong in anger over similar leafleting.

Police said they will determine when and where the video of last week's alleged launch was filmed, as well as the number of people involved.

"If we find any illegal activity, we will handle the case according to the law," a police official said.

Park's group has engaged in leafleting more than 60 times since the Ministry of Unification began keeping records in 2010. (Yonhap)