The United States recognises Japan's sovereignty over islands visited by Russian President Vladimir Putin, the US ambassador to Tokyo said Friday.

"The United States remains unwavering in its support of its most important ally and in its recognition of Japanese sovereignty over the Northern Territories," George Glass said on X, using the Japanese name for the Kuril islands visited for the first time by Putin on Thursday.

"At a time when the US-Japan Alliance is dedicated to cultivating prosperity across the Indo-Pacific, we strongly oppose any action that undermines its peace and stability," Glass added.

Russian-Japanese relations have long been dogged by the islands seized by the Soviet Union in 1945 at the end of World War II.

Putin's visit was the first by a Russian president since Dmitry Medvedev in 2010.

"The Northern Territories are an inherent part of Japan's territory, both historically and under international law," Japanese Prime Minister Sanae Takaichi said.

Japan summoned Russia's ambassador.

Russia's embassy in Japan said the ambassador had said the islands were "an integral part of the Russian Federation" and that Putin's travel there was not "a matter for discussion with foreign states".

A spokeswoman for the United Nations' Secretary-General Antonio Guterres, Shirin Yaseen, said "this is a bilateral issue between member states."