Senior Russian security official Dmitry Medvedev told Japan on Tuesday it would have to drop territorial claims to a group of Pacific islands if it wanted to conclude a peace treaty with Russia formally ending World War Two.

Deputy head of Russia's Security Council Medvedev marks WW2 victory over Japan.

The blunt remarks by Medvedev, a former president who is deputy chairman of Russia's Security Council, over what Moscow calls the Kuril islands are likely to anger Japan which lays claim to four of the southernmost islands, which it calls the Northern Territories.

Russia, the main successor state to the Soviet Union, and Japan have never signed a peace treaty formally ending their hostilities during World War Two, with the islands remaining the primary stumbling block.

The islands are located off Hokkaido, Japan's northernmost main island, and were seized by the Soviet Union at the end of World War Two.

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

But Russia withdrew from peace treaty talks with Japan and froze joint economic projects related to the islands in 2022 because of Japanese sanctions over Russia's war in Ukraine and relations have soured further since.

Medvedev said he was respondidipng to comments by Japanese Prime Minister Fumio Kishida who he said had spoken in favour of a peace treaty with Russia.

"Nobody's against the peace treaty on the understanding that ... the 'territorial question' is closed once and for all in accordance with the constitution of Russia," Medvedev said on his official X account.

In 2020, Russia's constitution was amended to bar handing over territory to a foreign power.

Medvedev, who styles himself as one of the Kremlin's most hardline anti-Western hawks, said Japan would also have to accept that Russia would develop the Kuril islands and station new weapons there.

"We don't give a damn about the 'feelings of the Japanese' concerning the so-called 'Northern Territories'. These are not disputed territories but Russia," said Medvedev.

"And those samurai who feel especially sad can end their life in a traditional Japanese way, by committing seppuku (Japanese ritualistic suicide by disembowelment). If they dare, of course."

Medvedev accused Japan of cosying up to the United States despite the fact that the U.S. military had dropped atomic bombs on Hiroshima and Nagasaki in 1945.

Russia said in December it regarded joint military exercises by Japan, the United States and Australia near Hokkaido to be a "potential security threat". It has complained about Japan - with U.S. help - expanding its military infrastructure and increasing arms purchases.

Japan has periodically expressed unease about Russia beefing up its military infrastructure on the disputed island chain.