Russian Prime Minister Dmitry Medvedev has angered Japan by paying another visit to the disputed Kuril Islands.
Mr Medvedev landed on Kunashir, which lies just north of the Japanese island of Hokkaido, Russia's government said.
Japan, which calls the four islands its Northern Territories, said the move was "extremely regrettable" and could have a negative impact on relations.
Tokyo wants the return of the islands, seized by Soviet troops in 1945 and held by Russia ever since.
The dispute over the islands has prevented the two countries signing a peace treaty formally ending World War II.
Mr Medvedev's first visit to the Kurils, as president in 2010, drew a furious reaction from Japan, which described it as an "unforgivable outrage".
He was the first Russian leader to visit the islands, and the move prompted Tokyo to recall its Moscow ambassador temporarily.
At its closest point Kunashir, the second largest of the island group, is less than 25km (15.5 miles) from Japan.
In his latest visit, Mr Medvedev spent a few hours on Kunashir at the head of a delegation of senior officials.
Mr Medvedev was due to inspect a range of industrial and social projects as well as meet local residents, Russian state media said.
Following the visit, Russia's ambassador in Tokyo was summoned to meet the Japanese vice foreign minister who told him that Kunashir was "an integral part of Japanese territory".
Tensions surrounding the Kurils spiralled after Mr Medvedev's 2010 visit, sparking fears of a conflict between the two countries.
Last year Russia announced it was boosting military defences on the islands.
However tensions reduced after the 2011 earthquake in Japan and the ensuing nuclear accident - a disaster which prompted expressions of solidarity in Russia.