North Korean leader Kim Jong-un's half-brother was killed at Malaysia's Kuala Lumpur airport last week.
"We believe the North Korean regime is behind this incident considering five suspects are North Koreans," said Seoul's unification ministry spokesman.
Malaysian police have already detained one North Korean and they say they are looking for four more.
Deputy national police chief Noor Rashid Ismail identified the latest North Korean suspects in a press conference on Sunday.
He said the men left Malaysia last Monday, the day Mr Kim was killed.
"The four suspects are holding normal passports, not diplomatic passports," he said.
Police believe poison was sprayed into Mr Kim's face he waited to board a flight to Macau.
Four other people have already been detained in connection with his death.
The four are: the North Korean, an Indonesian woman, a Malaysian man, and a woman with a Vietnamese passport.
The Indonesian national, named as Siti Aisyah, is said to have told Malaysian police she had been paid to perform what she thought was a prank.
Kim is believed to have been attacked in the airport departure hall on Monday by two women, using some form of chemical.
A grainy image taken from security camera footage, which has been broadcast in South Korea and Malaysia, shows a woman wearing a white T-shirt with the letters "LOL" written on the front.
Despite widespread speculation that North Korea was behind the killing, there has been no proof. Pyongyang has made no public comments on the issue.
Kim was largely estranged from his family, after being passed over for the North Korean leadership in favour of his youngest half-brother. He spent most of his time overseas in Macau, mainland China and Singapore.
He had spoken out in the past against his family's dynastic control of North Korea and in a 2012 book was quoted as saying he believed his younger half-brother lacked leadership qualities.
But he had said he was not interested in assuming the leadership himself.