The presence of US soldiers on Japanese soil is hindering closer ties between Tokyo and Russia and remains of concern to Moscow, Russian Foreign Minister Sergei Lavrov said Saturday.
There are some 54,000 American personnel stationed in Japan as part of a mutual security pact between the two countries, according to the US military.
Speaking on the sidelines of a G20 foreign ministers' meeting held in Nagoya, central Japan, Lavrov said these troops were "of course a problem on the road to improving the quality of Russia-Japanese relations."
He said Moscow's concerns "over its own security stemming from the presence, development and constant reinforcement of the US-Japan political and military alliance" had been transmitted to the Japanese side, who had "promised to react."
"We will await their response and continue discussions," said Lavrov.
The two countries are locked in negotiations to conclude a peace treaty they never signed to end hostilities after World War II.
Talks are deadlocked by a dispute over four islands, between the Sea of Okhotsk and the Pacific Ocean, seized by the Soviet army in the last days of World War II.
The string of volcanic islands are called the Kurils by Russia and the Northern Territories by Japan.
Japan considers the four islands part of their territory.
Last November, Russian President Vladimir Putin and Japan's Prime Minister Shinzo Abe agreed to accelerate talks towards formally ending their World War II hostilities.
They said they would resume talks on the basis of a 1956 declaration that restored diplomatic relations between Japan and the Soviet Union.
At that time, the USSR offered to give Japan the two smallest islands in exchange for agreeing to a treaty and Moscow keeping the bigger islands.
This deal was annulled by the Soviet Union in 1960 after Tokyo and Washington signed a cooperation accord.
"Let me remind you when the 1956 declaration was being negotiated, the USSR said that it could only be fully implemented if there was an end to US presence in Japan," said Lavrov.