Russian Foreign Minister Sergei Lavrov said Monday that he had summoned Japanese Ambassador to Russia Akira Muto after Japan protested Russian President Vladimir Putin’s visit last week to a disputed island off Hokkaido.

At a news conference, Lavrov said he called in the ambassador immediately after Prime Minister Sanae Takaichi and Foreign Minister Toshimitsu Motegi voiced their protest over Putin’s visit to the island on Thursday. The island, Etorofu, is one of four off the east coast of Hokkaido that are held by Russia and known as the Northern Territories in Japan.

According to Russia’s state-run Tass news agency, Maria Zakharova, director of the Russian Foreign Ministry’s Information and Press Department, said Monday that Muto would respond to the summons on Tuesday.

The report came after Lavrov said the Japanese Embassy in Moscow replied that Muto could not appear at the foreign ministry on Friday or Monday, the dates specified by the ministry, without providing a clear reason.

The Russian minister described the situation as completely unacceptable and a clear breach of protocol. He added that he would discuss with Muto not only Japan’s response to Putin’s visit to Etorofu but also how ambassadors should carry out their duties in the host country.

Takaichi and others protested Putin’s visit to Etorofu, which was his first visit to any of the four disputed islands. The islands are claimed by Tokyo but controlled by Moscow, after they were seized by the former Soviet Union from Japan at the end of World War II.

On Wednesday, Putin inspected Russian naval drills in Yuzhno-Sakhalinsk in the Russian Far East and reportedly claimed that international documents confirm that the Kuril Islands, including the four islands, are Russian territory.