U.S. State Department spokesman Philip Crowley said on Monday that the United States backs Japan on its islands dispute with Russian.

"We do back Japan regarding the northern territories," Crowley told reporters when asked to comment on the issue.

He said the United States has encouraged Japan and Russia to negotiate an actual peace treaty regarding the dispute and other issues.

Russian President Dmitry Medvedev on Monday morning visited Kunashiri Island, one of the Russian-held islands also claimed by Japan.

Medvedev becomes the first leader from Russia or the former Soviet Union to travel to any of the disputed islands, which are called the Northern Territories in Japan and the Southern Kurils in Russia.

The visit has sparked strong reaction from Japan. Upon Medvedev 's arrival on the island, Japanese Foreign Minister Seiji Maehara summoned Russian ambassador to lodge a protest against the visit.

The Russian envoy, however, insisted that the president's visit is Russia's domestic issue.

In return, Russian Foreign Minister Sergei Lavor said on Monday that Japanese ambassador will be summoned to the Russian Foreign Ministry over Tokyo's reaction to Medvedev's visit to Kuril Islands.

The minister told a press conference that Japanese reaction to Medvedev's visit was "unacceptable", adding that "this is our land and the Russian president visited Russian land."

The four disputed Pacific islands were occupied by the Soviet troops in 1945 and are currently under Russian control.

Russia and Japan have long been at odds due to the territorial dispute over these islands, which has blocked a peace treaty between the two countries since the end of World War II.