Russia said on Monday it had built new barracks for troops on a disputed chain of islands near Japan and would build more facilities for armoured vehicles, prompting a diplomatic protest.

Russia’s Ministry of Defence said it planned to shift troops next week into four housing complexes on two of the four disputed islands, known as the Southern Kurils in Russia and the Northern Territories in Japan.

Japan’s defence ministry says 3,500 Russian troops are deployed on the two larger islands as part of a military build-up. The news came after the Kremlin said Japanese Prime Minister Shinzo Abe might visit Russia on January 21 as part of efforts to defuse the territorial dispute that has prevented them from signing a World War II peace treaty.

In Tokyo, Foreign Minister Taro Kono told a regular news conference that Japan would lodge a protest, as reported by ABC News. Japan said in July it had asked Russia to reduce its military activity on the islands, a plea Moscow dismissed as unhelpful megaphone diplomacy at the time. “We plan to lodge a protest,” Mr Kono told reporters, adding that Japan would clearly state its position during negotiations.

Soviet forces seized the four islands at the end of World War II. Russia and Japan both claim sovereignty over them.