Japan and Russia have exchanged heated comments over the disputed southern Kuril islands in the Pacific Ocean.
Japan's PM Naoto Kan called a visit by Russia's president to the islands last year an "unforgivable outrage".
Mr Kan was speaking during a rally in Tokyo held to demand the return of the islands, which were occupied by Soviet troops at the end of World War II.
Russian FM Sergei Lavrov responded by saying that Mr Kan's remarks were "clearly undiplomatic".
Russian President Dmitry Medvedev became the first Russian leader to visit the islands last November.
The dispute over the islands, which lie north of Hokkaido and are known as the Northern Territories in Japan, has stopped Japan and Russia from signing a peace treaty.
Mr Kan was the main speaker at the Tokyo rally on Monday.
Japan has designated 7 February as Northern Territories Day, marking the anniversary of an 1855 treaty which Tokyo says confirms its claim. A protest rally was also held in Nemuro, Hokkaido.
Japanese Foreign Minister Seiji Maehara, who is to visit Moscow this week, has said he will stake his political career on securing the islands' early return.
Russia's response to Mr Kan's comments was swift.
"Those expressions are clearly undiplomatic," Mr Lavrov said.
"They contrast sharply with the respectful, positive tone that was characteristic of the meeting between Prime Minister Naoto Kan and President Dmitry Medvedev in Yokohama last autumn," Russia's top diplomat added.
He also accused Tokyo of supporting Japanese nationalist organisations, whose members reportedly burned a Russian flag during the demonstration in the Japanese capital.