Russian President Vladimir Putin said Thursday the sovereignty of two islands set under a 1956 agreement to be transferred to Japan on the conclusion of a peace treaty is still subject to future negotiations.

A day after holding talks with Japanese Prime Minister Shinzo Abe in Singapore, Putin noted that the 62-year-old joint declaration, which will serve as the basis for the forthcoming talks, does not explicitly specify the islands' sovereignty.

The declaration "does not say on what grounds and whose sovereignty they will fall under," Putin was quoted as saying in Singapore by Russia's Interfax news agency. "This is the issue to be scrutinized."