TOKYO (AFP) — Japan scrambled two dozen military aircraft and lodged a protest, accusing a Russian strategic bomber of entering its airspace over the Pacific Ocean south of Tokyo Saturday.

Russia denied the incursion, but the Japanese foreign ministry said it lodged a strong protest with the Russian embassy in Tokyo over the incident, which followed stepped up Russian long-range air patrols over the Atlantic.

"We have asked the Russian government to make a thorough investigation into the matter," a foreign ministry spokesman said.

The Tupolev Tu-95 bomber, which dates to the Soviet era, flew over the rocky isle of Sofugan, 650 kilometres (406 miles) south of Tokyo, for about three minutes from 7:30 am (2230 GMT Friday), the defence ministry said.

The air force scrambled 24 planes, including F-15 fighters and an E-767 radar plane, the defence ministry said.

They gave "a notice, then a warning and another a notice and a warning," a defence ministry statement said. "There was no response."

The Russian bomber then flew back north towards the Russian island of Sakhalin, it said.

Moscow said four Tupolev Tu-95 bombers completed a 10-hour mission over the Pacific on Saturday without violating Japanese airspace and that US fighters were also scrambled during the incident.