The US and Japan were forced to halt military exercises this week because of Russian jets flying nearby, Japanese officials have revealed.
Russia admitted its jets were flying above the Sea of Japan, but said the pilots had not breached any rules.
Moscow and Tokyo have argued recently over an island chain they both claim.
The US-Japan war games are the biggest ever held between the two countries, involving more than 44,000 personnel and 60 warships.
The drills - codenamed Keen Sword - are being held to mark the 50th anniversary of the US-Japan alliance.
Japanese government spokesman Yoshito Sengoku said the incident with the Russian jets took place on Monday.
"It is a fact that as part of the continuing Japan-US joint exercises, there was training taking place in the Sea of Japan on the sixth of this month," he said.
"It is also a fact that fighter jets were launched after Russian planes were detected over the Sea of Japan near Japan's Noto peninsula."
Roman Markov, a Russian military spokesman, said the jets were flying within their "zone of responsibility".
"The aircraft carried out a planned flight in an area of the Russian Pacific Fleet's regular activity," he said.
"Our pilots did not violate any rules of international airspace."
Russian President Dmitry Medvedev sparked a diplomatic row with Japan earlier this year by visiting the Southern Kurils, which Japan calls the Northern Territories.
The islands, off the north coast of Japan's Hokkaido island, were seized by Soviet troops at the end of World War II, but Japan still regards them as part of its territory.