TOKYO — Japan has lodged a protest against Russia over its move to lay undersea optical fibers linking the Far East and three disputed islands off Hokkaido, the top government spokesman said Monday.

Moscow had notified Tokyo of its plan to start laying such fibers on Sunday linking Sakhalin in the Russian Far East and the islands controlled by Russia but claimed by Japan.

Tokyo’s protest was lodged with a senior official from the Russian Foreign Ministry on Thursday, Suga said. The government also protested to China as a Chinese communications company is involved in the project.

“It’s extremely disappointing,” Chief Cabinet Secretary Yoshihide Suga said at a regular press briefing. “The government will continue to negotiate strenuously with Russia with an eye to resolving the issue of the attribution of the islands and concluding a peace treaty,” the top government spokesman said.

The islands of Etorofu, Kunashiri, Shikotan and the Habomai islet group, called the Northern Territories in Japan and the Southern Kurils in Russia, were seized by the former Soviet Union at the end of the war.