"The flag is unacceptable and very regrettable in light of Japan's position on the Takeshima islets’ sovereignty. In response to this case, we have delivered Japan's stance and expressed our strong protest to the South Korean side through diplomatic channels," Suga said.
In January, Seoul and Pyongyang agreed that the two countries’ national teams would march together under a "unification flag" at the Pyeongchang Olympics opening ceremony, following a thaw between two Asian nations prior to the world's championship.
The islands, which have also been known as the Liancourt rocks, have long been a roadblock in the diplomatic relations between the two states. South Korea claims they were annexed by Japan during its colonization of the Korean Peninsula in the 20th century.
The Unification Flag is an unofficial flag used to represent athletes from South Korea and North Korea as one team in sporting events. The flag was used for the first time when the two countries competed as a joint team in the 1991 World Table Tennis Championships in Japan and the World Youth Football Championship in Portugal. It was also used by the national teams of the two Koreas during the opening ceremonies of 2000, 2004 and 2006 Olympics.