Russia and Japan’s territorial dispute deepened Thursday after Tokyo sent a note of protest to Moscow over the latter’s plans to deploy troops to the contested Kuril Islands in the Pacific Ocean, according to reports. Russia controls the Kuril Islands, but Japan has laid claims to the southern part of the islands.

Japanese Chief Cabinet Secretary Yoshihide Suga said his country was keeping a close watch on the Russian movement. "If the move leads to the reinforcement of Russian military on the islands, it would be incompatible with Japan's stance and it is regrettable as they are inherently our territory," Suga said. "This is contrary to the position of our country.”

On Wednesday, Russian Defense Minister Sergei Shoigu announced that Moscow "was continuing active work to defend the Kurils." He added his country "should deploy a division" there this year.

Russia later clarified that it does not plan to use the military on the islands against Japan. "We do not intend to use our armed forces against Japan by deploying them to the Kuril Islands … We are ready to cooperate, including in the military sector, in the sphere of countering terrorism and theft of marine biological resources," Viktor Ozerov, chairman of the defense committee in the Russian upper house of parliament, told Sputnik Thursday.

Moscow's investing in military infrastructure on the Kuril Islands has strained relations between the two nations. Last June, Russia announced its plans to equip its military infrastructure on the Kurils with an advanced Strelets-Chasovoy security system.

The dispute surrounds the four southernmost islands in the Kuril chain, known in Japan as the Northern Territories. The islands fell under Soviet jurisdiction at the end of World War II. Moscow and Tokyo did not sign an official peace agreement at the end of the war because of the dispute, which also dampened trade ties between the two countries for decades.