A Japanese group's plans to explore for Russian oil with state-run Rosneft PJSC have been stymied by U.S. intervention over sanctions, according to people familiar with the matter.

Washington's objection to the Japanese project to explore for oil in the ocean off the Russian Far East shows the U.S. Treasury is maintaining a firm line on sanctions, even as some international companies press on with Russian energy deals. In April, the U.S. turned down a request from Exxon Mobil Corp. for a waiver to allow it to drill with Rosneft in the Black Sea.

Rosneft signed a preliminary deal with a consortium of Japan Oil, Gas & Metals National Corp., known as Jogmec, Inpex Corp., and Marubeni Corp. for offshore exploration at a license to the southwest of Sakhalin Island in December, one of more than 60 agreements and memorandums signed during Russian President Vladimir Putin's visit to Japan.