Dmitry Medvedev (left) attends the all-Russian youth educational forum "Iturup" in Kurilsk during his visit to Iturup island, on August 22, 2015
Moscow (AFP) - Russian Prime Minister Dmitry Medvedev on Saturday visited the Kuril islands, prompting a swift rebuke from Tokyo, which claims sovereignty over the archipelago in a long-running dispute.
Medvedev went to Iturup, one of four islands in the chain that lies off Russia's far eastern coast and just north of Japan, for a day-long visit which included a photo-op with a giant Russian flag and overseeing military training.
He also chose the time of the visit to make public a government decree on expanding Russia's shelf in the Sea of Okhotsk, a marginal sea which lies just north of the Kurils.
Medvedev said Japan was "getting upset for nothing".
The visit has led to Japanese Foreign Minister Fumio Kishida postponing a planned trip to Russia in the next few weeks, Japan's Kyodo news agency reported, quoting government sources.
Kishida had been expected to prepare the ground for Russian President Vladimir Putin to make a long-delayed visit to Japan this year to seek a breakthrough in the 70-year-old territorial dispute.
Japan called in Russia's ambassador to Japan in protest at the visit to the islands, which Japan calls the Northern Territories.
Hajime Hayashi, the head of the Japanese foreign ministry's European division, said Medvedev's trip "contradicts Japan's position over the Northern Territories and hurts the feelings of the Japanese people... It is extremely regrettable".
Medvedev however dismissed Japan's concerns, saying that Russian officials "have visited, are visiting, and will visit the Kurils."
"We want to be friends with Japan, Japan is our neighbour... but this has nothing to do with the Kuril islands which are part of Russia and are in a Russian region called the Sakhalin region."
"Our colleagues are getting upset for nothing. This is how it is and how it will be," he said, as quoted by Russian agencies.
He added that a decision has been reached to base a "modern effective military force" on the islands.
The defence ministry is currently building hundreds of residential buildings for the military on Iturup and neighbouring Kunashir (known as Etorofu and Kunishiri in Japan), with soldiers expecting to move there next year.
Soviet troops seized the islands just after Japan surrendered in World War II.
The seven-decade-old dispute has hampered trade and prevented Moscow and Tokyo from signing a formal post-war peace treaty.
Both the Kremlin and Japanese Prime Minister Shinzo Abe had hoped to start mending relations in order to revive trade, with Japan seeking broader access to Russia's plentiful oil and natural gas supplies.
According to the government's website, Medvedev visited the island's new airport, sea port, a fish processing factory, and a youth forum where he met with young scientists and teachers.
He and the forum participants took selfies and unfurled a giant Russian flag on the island's shore for a group picture to mark Russia's flag day, according to Medvedev's Instagram account.
"At the Kurils on Russia's State Flag day," the picture was captioned.
He went to the Kurils after making an official visit to Russia's Far East region of Amur, where he visited a space port being built in Vostochny.
The cosmodrome is designed to ease Russia's dependence on space launches in Baikonur, in Kazakhstan.
Medvedev visited the islands in 2012, and Russia held military exercises there in 2014. Both incidents provoked protests from Tokyo.