Moscow, July 27 : Despite Japan's opposition, Russian Prime Minister Mikhail Mishustin visited the disputed island of Iturup, known as Etorofu in Japanese, triggering a new round of tensions between Moscow and Tokyo. On Monday, Mishustin inspected a hospital and talked with representatives from a fish processing plant on Iturup, one of the four Pacific islands claimed by both countries, called the Southern Kurils in Russia and the Northern Territories in Japan, reports Xinhua news ageny. The Prime Minister said that he had discussed with President Vladimir Putin possible measures that can be taken on these islands, which are now under Russia's control, to boost investment and economic activities. He said that Russian authorities are considering to set up a free trade zone on the contested islands to create opportunities for imports, which would be "a good decision for many investors". The four islands used to belong to Japan, which were seized by the Soviet Union after World War II and were later incorporated into Russia following the collapse of the latter. Tokyo maintains that Moscow illegally occupied the islands. The Japanese Foreign Ministry said in a statement on Monday that Mishustin's visit to Iturup island was "extremely regrettable" and urged the Russian side to "take constructive measures" to advance Tokyo-Moscow ties. The Ministry summoned Russian Ambassador Mikhail Galuzin and issued a strong protest. In response, the Russian Foreign Ministry summoned Japanese Ambassador Toyohisa Kozuki in a retaliatory move. Kremlin spokesman Dmitry Peskov said that Mishustin has every right to visit Russian territories, while at the same time Moscow has the strong political will to develop relations with Japan. /IANS