Prime Minister Naoto Kan told a briefing in Tokyo that Medvedev’s trip in November to what Russia calls the Southern Kurile islands, in the western Pacific, was “an unforgivable outrage.”

Russia’s Foreign Minister Sergei Lavrov called Kan’s remarks “clearly undiplomatic.”

Lavrov also hit out at the Japanese leadership for “deciding not to distance itself” from nationalist civil society groups, whose actions he called “absolutely unacceptable.”

Ultra nationalist groups orchestrated “disgusting public acts desecrating” the Russian flag in front of the Russian embassy in Tokyo on Friday, Russia’s Foreign Ministry said in a statement.

“We must ascertain that the present surge in confrontational demonstrations in Japan is a result of the authorities’ policies,” the foreign ministry said, accusing the Japanese government of funding anti-Russian groups.

“We have to state that the recent spike of confrontational acts in Japan is the result of the policy of this country’s authorities. In particular, the Japanese government is every year increasing funding for so-called ‘movement for the return of the northern territories’.”

Medvedev’s trip was the first by a Russian leader to the Russian islands, which have been a persistent irritant in relations.

The Soviet Union occupied the four disputed islands at the end of World War Two and the territorial row has weighed on relations between Tokyo and Moscow ever since, preventing the signing of a formal peace treaty.

Japan refers to the islands as its Northern Territories.

“Russia’s sovereignty over the Kurile Islands is not up for any kind of review, not today, not tomorrow,” top Kremlin aide Sergei Prikhodko was quoted by Russian news agencies as saying.

The accusations threatened to erode bilateral ties just before a visit by Japanese Foreign Minister Seiji Maehara to smooth relations later this week.

Moscow has pointedly asserted sovereignty over the disputed small, windswept islands off Russia’s Pacific Coast with the visits by Medvedev and two senior government officials in recent months — sparking a flurry of angry responses from Japan.