Seoul said the announcement proved Pyongyang was behind the murder of Kim Jong-Nam, who died after being squirted in the face with an unidentified liquid at Kuala Lumpur International Airport on Monday.
The case has also sparked a diplomatic row between Pyongyang and Kuala Lumpur, after Malaysia rejected demands quickly to hand the body over to the North.
Four North Korean men were being sought over the killing, Deputy Inspector-General of Police Noor Rashid Ibrahim told a press conference, in addition to their 46-year old compatriot Ri Jong Chol who was arrested in Kuala Lumpur Saturday.
Three more North Koreans were wanted for questioning, he said.
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Officers have already arrested one North Korean, an Indonesian woman and her Malaysian boyfriend, as well as a Vietnamese woman.
"Considering that five suspects are North Korean nationals, we view that the North Korean government is behind the incident," Seoul's unification ministry spokesman Jeong Joon-Hee said immediately after today's announcement.
The deputy police chief refused to comment on any political motive for the killing, saying only that investigations were ongoing.
"We are trying very hard to get the next of kin to come and assist us in the investigation," Noor Rashid said, but added no such family member had yet come forward.
Police were still waiting for the results of an autopsy conducted on Wednesday, he said.
Vietnamese suspect Doan Thi Huong, 28, worked as an "entertainment outlet employee", police said Sunday, while 25-year-old Indonesian Siti Aishah was a masseuse at a spa.
"She has always been very fashionable, with colourful hair," neighbour Maria Nguyen told AFP, adding that every lunar new year Huong would return to the village with a different foreign boyfriend.
Huong was seen in CCTV images at Kuala Lumpur airport wearing a white top with the letters "LOL" emblazoned on the front, according to Malaysian media.
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