U.S. President Barack Obama, during telephone talks Feb. 9, expressed concern about Prime Minister Shinzo Abe’s possible visit to Russia in early May, according to diplomatic sources.

Obama said he wants Abe to postpone his visit to Russia until after the Group of Seven summit in Japan in late May, the sources said Tuesday.

Abe was quoted as responding that Japan and Russia have important issues to resolve.

The telephone conversation was held after North Korea’s rocket launch earlier this month.

Abe is scheduled to visit Europe for talks with his G-7 counterparts in the region from late April through early May. He is also eying a stopover on the way back in Sochi, southwestern Russia, to have talks with President Vladimir Putin.

Through the possible unofficial visit to Russia, Abe is eager to kick-start bilateral territorial talks on the four Russian-held islands off Hokkaido.

Abe also plans to call for Russia’s cooperation in dealing with North Korea, and urge restraint by Moscow over the crisis in Ukraine.