The foreign ministers of Japan and Russia will head a new framework of negotiations to conclude a peace treaty, but sovereignty over disputed isles remains a major hurdle in progress toward that goal.

Prime Minister Shinzo Abe reached an agreement on the new framework in talks with Russian President Vladimir Putin on Dec. 1 in Buenos Aires on the sidelines of the Group of 20 summit.

“We agreed to create a supplementary mechanism for future mutual work as well as to heighten the level of trust,” Putin said at a news conference after his meeting with Abe.

The latest move follows the Nov. 14 agreement in Singapore between Abe and Putin to quicken the pace of negotiations for a peace treaty based on the 1956 Joint Declaration between the two countries.

Until now, Abe has used various special aides to negotiate with Russian officials to achieve a breakthrough on the dispute over the Northern Territories, the four small islands off the coast of Hokkaido that were seized by Soviet forces at the end of World War II.

But with the focus now on concluding a peace treaty, Foreign Minister Taro Kono and his Russian counterpart, Sergey Lavrov, will be in charge of the negotiations pertaining to treaties that come under the purview of the Foreign Ministry.

The nuts-and-bolts negotiations will be handled by Takeo Mori, a senior deputy minister for foreign affairs who will serve as special representative for Abe, and Igor Morgulov, a Russian deputy foreign minister.

According to government officials who briefed reporters, Kono and Lavrov will hold a meeting under the new framework before Abe visits Russia in January.

Further negotiations would be handled by the two foreign ministers and their close aides, with the goal of reaching an agreement in principle by June 2019, when the G-20 summit is scheduled to be held in Osaka and with Putin expected to attend.

Japan had long insisted that Russia return all four disputed islands before a peace treaty could be signed.

But the Singapore agreement signaled Abe’s intent to focus negotiations on first having Russia return two of the isles--the Habomai islets and Shikotan--as spelled out in the 1956 declaration.

However, at a news conference in Singapore immediately after his meeting with Abe, Putin said the 1956 declaration does not clarify which nation would have sovereignty over Habomai and Shikotan.

The difference over sovereignty is not the only sticking point in negotiations with Russia.

The increasingly tense relations between Russia and the United States, caused in part by the recent seizure of Ukrainian ships by Russia, could also cast a large cloud over Japan-Russia talks.

Abe has told Putin that if Habomai and Shikotan were returned to Japan, no U.S. military bases would be allowed on those islands.

However, Moscow might not be convinced about that pledge, given that the United States could reject any restrictions on how it wants to implement the joint security treaty with Japan.

Japan and Russia have tried various channels to resolve the territorial dispute and sign a peace treaty with little to show in the way of results.

One such example came in 1998, when the foreign ministers of the two nations co-chaired a joint committee to discuss the territorial dispute. However, that committee failed to resolve the issue.

Moreover, confrontations between Russia and the United States have also hampered Tokyo-Moscow negotiations in the past.

After Russia annexed Crimea in 2014, Japan joined the United States and European nations in slapping economic sanctions on Russia. But the international criticism directed at Russia did not stop Japan from continuing to hold negotiations with Russia over the Northern Territories.

Still, the special consideration given to Russia apparently did not please Putin. He brought up the sanctions issue in October and said it was not a measure that contributed to fostering trust between Russia and Japan.

(This article was compiled from reports by Kotaro Ono and Takashi Kida in Buenos Aires and Yuka Takeshita in Tokyo.)