Russian President Vladimir Putin has visited the Kuril Islands, a Pacific island chain seized by the Soviet Union from Japan in 1945, drawing immediate protest from Tokyo. Japanese Prime Minister Sanae Takaichi called the visit "unacceptable," saying it violates Japan's position on the islands, which it claims as its Northern Territories. The dispute over the islands has prevented Russia and Japan from signing a formal peace treaty after World War II. Takaichi, a public critic of Putin, has faced his denunciations over Japan's sanctions against Russia for the war in Ukraine. Putin has repeatedly criticized those sanctions. The visit, reported by both Al Jazeera and The New York Times, marks a fresh escalation in the long-standing territorial row. Japan's government issued a formal protest, though further details were not immediately available. The islands, known as the Kurils in Russia and the Northern Territories in Japan, have been under Moscow's control since the Soviet takeover at the end of the war. Tokyo maintains that the islands are inherent Japanese territory, while Russia insists its sovereignty is indisputable. The timing of Putin's visit was not specified in the reports.