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 armored vehicles, prompting a diplomatic protest from Tokyo.

Russia's Ministry of Defense 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 defense ministry says 3,500 Russian troops are deployed on the two larger islands as part of an on-going military buildup.

The news came after the Kremlin said Japanese Prime Minister Shinzo Abe might visit Russia on Jan. 21 as the two countries step up 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. 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.

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

Diplomats on both sides have spoken of the possibility of reviving a Soviet-era draft agreement that envisaged returning two of the four islands as part of a peace deal.

President Vladimir Putin and Abe have held numerous face-to-face meetings to try to make progress on the issue.