TOKYO—China's deal Wednesday to buy Russian natural gas points to the advantage it holds over East Asian rival Japan, which also wants Russian energy but has to accommodate the concerns of its ally, the U.S.

Japan's government is a partner with OAO Rosneft in an energy venture called Sakhalin-1 in the Russian Far East. U.S. sanctions have targeted Rosneft's chief, Igor Sechin, although not the company itself. Japan also relies on the nearby Sakhalin-2 field in Russian waters for about 10% of its supplies of natural gas, putting Tokyo at risk should the U.S. ratchet up pressure on Moscow.

Energy specialists say Japan's commitments to the Russian projects—and its strong demand for fossil fuels in the wake of its 2011 nuclear accident—make it hard to pull back now.

"Russia is looking even more to Asia than before. If Japanese companies hesitate to do business, it would be China that picks up the windfall gains," said Nobuo Tanaka, a former executive director of the International Energy Agency who is now a professor at Tokyo University.

Still, Japan's alliance with the U.S. makes it harder to woo Russia unreservedly.