By Denis Dyomkin
YUZHNO-SAKHALINSK, Russia (Reuters) - Russian Prime Minister Dmitry Medvedev on Tuesday landed on a remote island chain seized from Japan by the Soviet Union at the end of World War Two, prompting protests from Tokyo which lays claim to the windswept archipelago.
Medvedev's trip to the disputed islands in Russia's Far East - known as the Southern Kuriles in Russia but as the Northern Territories in Japan - saw Japan's Ministry of Foreign Affairs summon Russia's envoy to complain.
"(This) is a territory inherent to Japan, therefore the visit of this kind is unacceptable for Japan and deeply regrettable," Vice-Minister for Foreign Affairs Kenichiro Sasae told Russian Ambassador Yevgenny Afanasiyev.
The islands lie off Russia's eastern coast, some 7,000 km (4,350 miles) from Moscow, and the island of Kunashir, where Medvedev landed, is a mere 15 km away from Japan's northern island of Hokkaido. Moscow's plans to boost investment in the region's infrastructure and industry have angered Tokyo.
"The islands of the Kuriles are ... our territory which should develop just like the mainland of our country," Medvedev told reporters in the city of Yuzhno-Sakhalinsk before departing by plane to Kunashir.
"We have to develop new investment projects including those with the participation of foreign firms."
Medvedev visited a fishery and said he planned to speak with members of the local government.
He was the first Russian leader to visit the islands during a trip as president in 2010. After that visit, he promised to bolster an artillery division deployed on the islands.
Russia has dedicated new funds and political attention to boost Moscow's presence in the country's vast but sparsely populated Far East in advance of an Asia Pacific Economic Cooperation (APEC) summit in Vladivostok in September.
Since Vladimir Putin's return to the presidency this year, it has also created a new ministry of the Far East, whose chief Viktor Ishayev traveled with Medvedev to the island chain along with the Minister of Regional Development Oleg Govorun.
Japan, which said Medvedev's trip had put a "damper" on a positive atmosphere between Japan and Russia, is planning to send the foreign minister to Moscow this summer to "revive" talks on the territorial issue.
Plans for that visit were firmed up after a meeting between Prime Minister Yoshihiko Noda and Russian President Vladimir Putin, which took place on the sidelines of the G20 summit in Mexico last month.
Russia's Chief-of-Staff Nikolai Makarov has said Russia may send Mistral-class helicopter carriers it is buying from France to the Pacific to help protect the islands though some analysts believe the move is actually aimed at boosting the country's defenses against the rising strength of neighboring China.
Unlike Japan's dispute with China over islands in the East China Sea, which are near potentially vast maritime oil and gas reserves, the feud with Russia has more to do with the legacy of World War Two.
Soviet soldiers seized the islands at the end of World War Two and the territorial row has weighed on diplomatic relations between the countries ever since, precluding a formal peace treaty.
The dispute remains an emotional one in Russia, especially in nationalist circles.
(Additional reporting by Tetsushi Kajimoto in Tokyo; Writing by Thomas Grove; Editing by Andrew Osborn)