Prime Minister Shinzo Abe has snubbed an appeal by U.S. President Barack Obama, who asked that he not visit Russia in May, sources close to Japan-Russia relations said Tuesday.

Obama made the appeal by phone on Feb. 9, but Abe rejected it and will press ahead as planned with a visit to the Russian city of Sochi for talks with President Vladimir Putin, likely in early May, the sources said.

Obama delivered the appeal in a conversation primarily about cooperation between Tokyo and Washington over North Korea's launch of a rocket two days earlier. The Foreign Ministry has not publicly revealed what the two leaders discussed regarding Abe's trip to Russia.

The sources quoted Obama as urging Abe to postpone the trip, citing Russia's differences with the United States over its actions in Ukraine and Syria.

Abe is keen to settle a long-standing dispute over islands off Hokkaido seized by the Soviet Union after Japan's surrender in World War II. The matter has prevented Moscow and Tokyo from signing a peace treaty.

The islands are Etorofu, Kunashiri, Shikotan and the Habomai group of islets, although Russia knows them by other names and refers to them as the southern Kurils.

"Prime Minister Abe is planning an unofficial visit to Russia at an appropriate time before (Russian President Vladimir Putin) visits Japan," Chief Cabinet Secretary Yoshihide Suga said at a news conference Wednesday.

Political observers say Japan has been careful to maintain a delicate balancing act between Russia and its ally the United States while pushing to end the isles dispute.