Japan's Prime Minister Naoto Kan on Monday called Russian President Dmitry Medvedev's visit three months ago to a disputed Pacific island chain an "unforgivable outrage", media reports said.
Kan also said Tokyo would tenaciously seek a settlement of the row over the Southern Kurils, known as the Northern Territories in Japan, which has prevented the signing of a post-World War II peace treaty, Kyodo News said.
The long-simmering row flared up again after Medvedev in November became the first Russian leader to visit the windswept, volcanic-origin islands located between Japan's Hokkaido island and Russia's Kamchatka peninsula.
Other Russian ministers have followed, despite repeated and strong diplomatic protests from Tokyo. Most recently, Defence Minister Anatoly Serdyukov inspected Russian military facilities there last Friday.
Japan's centre-left premier Kan said Monday: "I will patiently negotiate, with a strong will, in accordance with the basic policy to solve the territorial issue of the four islands and to sign a peace treaty."
"The issue of the Northern Territories is an extremely significant subject for Japan's diplomacy," Kan told the annual National Meeting to Demand the Return of the Northern Territories, Jiji Press reported.
Japanese Foreign Minister Seiji Maehara, who is due to visit Moscow from Thursday, said that he would work toward the return of the islands, "even if it costs me my political career".
He also pledged an "all-out effort to convey Japan's position" to Russia, and said he hoped for a solution to the row in a future Kan-Medvedev summit.
The dispute with Russia flared up last year at a sensitive time for Japan, when it was also embroiled in a territorial row over islands with China.
The disputed Kuril islands have been controlled by Moscow since they were seized by Soviet troops in the final days of World War II, with their Japanese residents expelled and replaced by Russian settlers.
Japan has since demanded the return of the southernmost four islands.
Northern Territories Day, attended by 1,600 former islanders, activists and politicians on Monday, marks the conclusion of an 1855 treaty with Russia which confirmed that the four islands are Japanese territory, Tokyo says.
Now home to around 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.
Medvedev in a speech in December said that Japan should realise that Moscow would not give up the four disputed islands, proposing instead to consider creating a joint free economic zone there.
Last Friday, Russian Defence Minister Serdyukov said he visited the islands to ensure that the region's machine gun and artillery division was fully equipped with the latest issue of weapons.
Maehara summoned Russian ambassador Mikhail Bely for a strong protest.
Russia's foreign ministry said Saturday it was "disappointed" at Japan's furious reaction to the defence minister's trip, and indicated it was becoming bored with the regular Japanese complaints.
"The Russian foreign ministry has already reacted several times to these comments (and) we are disappointed by the fact that the Japanese side continues" to make them, the foreign ministry said.
Medvedev tried to defuse the dispute Friday by telling a Russian Security Council meeting that Serdyukov's visit was not meant as a show of force.
But he stressed once again that the islands "are Russian territories that must develop according to a very obvious scenario; just like all the other regions of Russia."