Japan's peace treaty negotiations with Russia based on a 1956 joint declaration involve the issue of "the attribution of four islands" under dispute between the two countries, Prime Minister Shinzo Abe said Friday.

Abe made the remark two days after he agreed with Russian President Vladimir Putin at their meeting in Singapore that the declaration -- which stated the return of Shikotan and the Habomai islet group upon the conclusion of a postwar peace treaty -- will be the basis for forthcoming negotiations.

Speaking at a news conference in the northern Australian port city of Darwin, Abe said Wednesday's agreement to accelerate talks based on the declaration would "not contradict" Tokyo's long-standing policy of resolving the issue of the status of the four islands before signing a peace treaty.

His comments suggest that even if Japan and Russia agree on the return of two smaller islets, Tokyo intends to continue negotiations for the attribution of two other islands -- Kunashiri and Etorofu -- and conclude the treaty.

"Under the leadership of me and President Putin, I'm determined to complete peace treaty negotiations, an issue of concern that has been left after the end of the war," the prime minister said, in reference to World War II.

The decades-old territorial dispute involves what Japan calls the Northern Territories, the four islands off Hokkaido that the former Soviet Union seized following Japan's surrender in 1945.

They are known as the Southern Kurils in Russia.

On Thursday, Putin, during a press conference in Singapore, raised ambiguity about the ownership of the two islands, even if they are officially returned to Japan based on the joint declaration.

While acknowledging the legal validity of the declaration, Putin suggested that the question of their sovereignty will still be subject to future negotiations.

He was quoted by Russian media as saying the declaration "does not say on what grounds and whose sovereignty they will fall under."

Besides speaking about the territorial issue, Abe said at Thursday's press conference that he will soon instruct government officials to compile a second supplementary budget for fiscal 2018 through March 31 as part of efforts to bolster the economy.

He also said a budget for next fiscal year will include sufficient measures to cushion possible adverse effects from a planned increase in the consumption tax next October.