A group of South Korean lawmakers on Saturday visited the disputed Takeshima islets in the Sea of Japan, adding fuel to ongoing spats over trade and historical issues between Tokyo and Seoul.

South Korea’s YTN television network broadcast the visit to the islets, known as Dokdo in South Korea. The islets are controlled by Seoul but claimed by Tokyo. The lawmakers said on Friday they would visit to protest Japan’s claim.

The move comes at a time when the two countries’ relations have hit their lowest point in years following a series of South Korean court rulings ordering compensation for people who said they were victims of forced labor during Japan’s colonial rule of the Korean Peninsula from 1910 to 1945 and Japan’s move to impose trade restrictions.

Later Saturday, Tokyo lodged a complaint with South Korea.

Kenji Kanasugi, director-general of the Foreign Ministry’s Asian and Oceanian Affairs Bureau, told Kim Kyung-han, political minister at the South Korean Embassy in Tokyo, that the islands are “an inherent part of Japanese territory in light of historical facts and international law” and such a visit is “unacceptable,” according to the Japanese Foreign Ministry.

Last month, Tokyo lodged a similar complaint following South Korean military drills on and around the islets.

During the drills conducted Aug. 25 and 26, participants from the ground, air and naval forces and maritime police practiced operations to protect the rocky islets. The size of the drill was nearly double its usual scale.

The exercise began just three days after South Korea announced its decision to terminate a military information-sharing pact with Japan in response to Tokyo’s export restrictions on Seoul-bound high-tech chemicals necessary to manufacture semiconductors and smartphone displays.