|The long-standing dispute has raised tensions with nationalists in Japan [Reuters]
Naoto Kan, Japan's prime minister, has called for Russia to return four disputed islands, held by Russia since the end of World War Two, prompting an angry response from Moscow.
Speaking at a rally in Tokyo on Monday, attended by hundreds of former Japanese residents of the islands, Kan called a recent visit to the area by Dmitry Medvedev, the Russian president, an "unforgivable outrage".
The islands, known as the Kuril's in Russia and as the Northern Territories in Japan, are the cause of a long-simmering row between the nations.
The issue was brought into focus in November last year after Medvedev became the first Russian leader to visit the region, located in the Pacific.
His unprecedented trip was followed by a series of such visits to the chain by other Russian officials.
Kan told the rally - the annual National Meeting to Demand the Return of the Northern Territories - that Tokyo would seek a settlement to the dispute.
"The issue of the Northern Territories is an extremely significant subject for Japan's diplomacy," Kan told the crowd, according to Jiji Press.
In Moscow, Sergei Lavrov, Russia's foreign minister, called Kan's comments "undiplomatic".
He accused the Japanese prime minister of pandering to nationalist interests and noted that any talks over the islands must be based on Tokyo's "unconditional recognition of the outcome of World War Two".
The rally in Tokyo had received broad media play in Moscow because it featured the burning of a Russian tricolour flag.
The sharp diplomatic exchange has cast a pall over what had already promised to be a delicate meeting next Friday in Moscow between Lavrov and Seiji Maehara, his Japanese counterpart.
Home to about 19,000 people, the islands are rich in gold and silver and lie in waters abundant in marine life, but their infrastructure is crumbling and most Russian residents eke out a meagre living there.