TOKYO, Japan — Two Russian planes entered air space above the Sea of Japan where U.S. and Japanese troops were conducting joint drills Monday, causing a temporary halt to the exercise, U.S. and Japanese officials said Wednesday.

Japanese fighters scrambled in response, Deputy Cabinet Secretary for Public Relations Noriyuki Shikata said during a press briefing at the Prime Minister’s office. He did not say whether there was any interaction between the Japanese and Russian aircraft.

The U.S.-Japan training — part of the bilateral Keen Sword exercise now under way in the skies and waters surrounding Japan — was briefly stopped because of safety risks posed by Russian IL-38’s entering the maneuvering space, U.S. military spokesman Lt. Col. Kenneth Hoffman said. The IL-38 is a maritime patrol and anti-submarine warfare aircraft.

Because the exercise was taking place in international airspace, the move by Russia was “not unexpected” and did not significantly affect training operations, Hoffman said.

It is the second incident in as many months that some have interpreted as a potential strain on Japan-Russia relations.

In November, Russian President Dmitry Medvedev visited outlying islands claimed by both countries, which the Japanese Foreign Minister Seiji Maehara reportedly toured by a plane last week.