The website published photos and footage showing Mr Choe reading his arrival statement at Pyongyang's worldwide airport.
Choe is the son of the former foreign minister Choe Dok-shin, who defected with his wife to North Korea in 1986 after political disputes with the then president, Park Chung-hee. Choe Tok-sin died in 1989 and Ryu Mi-yong assumed his role as leader of a religious sect.
Mr Choe said he is more than 70 years old and has chose to defect because it was his parents' "dying wishes" for him to "follow" North Korea and work for its unification with the South, according to Uriminzokkiri.
South Korea's unification ministry said Choe In-guk was in North Korea without special permission from the Seoul government to visit.
According to South Korea's Unification Ministry, Choe In-guk has been allowed to make 12 authorized trips to North Korea since 2001 for events such as visiting his parents' cemetery and attending a death anniversary for his mother.
Choe In-guk's father Choe Tok-sin fell out with South Korea's strongman president Park Chung-hee in the 70s and sought to emigrate to the United States in 1976.More news: Every free NES game added to Nintendo Switch Online in July 2019
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The government is trying to find out the exact motive for the 73-year-old Choe's trip.
A rare case of a South Korean citizen seeking to settle in the North occurred after a man in his seventies fled to North Korea, BBC reported Monday. It is more common for people from the North to try to escape to the South.
South Korea's decision to strengthen maritime border security includes plans to deploy a mid-sized warship, and several maritime patrol helicopters near the NLL.
South Koreans have occasionally defected to North Korea in the past, but it's become a rarity in recent years - especially since the North suffered a crippling starvation in the mid-1990s that was estimated to have killed hundreds of thousands of people.
The North's propaganda Web site Uriminzokkiri said on Sunday that Choe In-guk, the second son of Ryu Mi-yong, the late chairwoman of the central committee of the Chondoist Chongu Party, arrived in Pyongyang for "permanent residence". Choe's parents are said to be buried in Pyongyang's Patriotic Martyrs cemetery.
Seoul's statement came a day after Japanese Prime Minister Shinzo Abe said on local TV that it's clear Seoul is not keeping its global agreement with Tokyo over the issue of wartime forced labor, .so it's reasonable to think that South Korea is not complying with trade restrictions against North Korea.
Choe's late father, Choe Deok-sin, worked as South Korea's foreign minister and an ambassador to West Germany in the 1960s, The New York Times reported.