Russian President Vladimir Putin on Thursday expressed concern that the disputed islands controlled by Moscow but claimed by Tokyo could be armed by the U.S. military if they are returned to Japan.

Putin also said in an interview with Kyodo News and other media outlets that Russia’s military buildup on the isles off Hokkaido is aimed at countering U.S. initiatives such as missile defense.

While emphasizing that demilitarization on the islands is possible, Putin said it should be considered in the context of the alleviation of tension in the region across the board.

The islands of Etorofu, Kunashiri, Shikotan and the Habomai islet group were seized by the Soviet Union after Japan surrendered at the end of World War II in August 1945. The dispute has prevented the two countries from signing a postwar peace treaty.

Putin is visiting St. Petersburg, his hometown, to attend the annual International Economic Forum, which started Thursday in Russia’s second-largest city.