Three Japanese historians on Thursday visited the South Korean-controlled Takeshima islets claimed by Japan to carry out on-site research, a member of the group said.
While dozens of people, including Japanese, are believed to visit Takeshima, called Dokdo in South Korea, for sightseeing every year, it is unusual for a trip to the disputed islets in the Sea of Japan to be publicly announced.
The three include Norio Kuboi, deputy director of an organization opposed to Shimane Prefecture's designation of a day commemorating the islets' incorporation as Japanese territory more than a century ago.
After crossing over from nearby Ulleungdo Island, they spent about 20 minutes on the small rocky outcroppings, assessing the conditions there and taking photographs.
Speaking by telephone, Kuboi said: "We confirmed that South Korean residents are living on Takeshima, and that the islets are being guarded. It is clear that South Korea is in a position of being able to claim sovereignty over the islets."
The Japanese Consulate in Busan earlier this week called on the group to cancel the trip. The Japanese Foreign Ministry calls on Japanese to refrain from visiting Takeshima, saying such visits "serve to recognize South Korean jurisdiction over the islands."
On Tuesday, while in Busan, the group said the trip was part of its investigation into the circumstances of the islets' 1905 incorporation into Shimane Prefecture.
The group originally had four members, but just prior to departure one of them disputed Seoul's sovereignty over the islets and was refused boarding.
On Tuesday, Kuboi said, "Japan incorporated Takeshima into Shimane Prefecture in order to prosecute the Russo-Japanese War, and this led to the invasion of the Korean Peninsula.
"The Japanese government's assertion that a territorial dispute exists over Takeshima is an attitude lacking remorse for the invasion and whitewashing it."