USNS Tippecanoe (C) refuels Japan Maritime Self Defence Force destroyers Ikazuchi (R) and Kongo during their military manoeuvres known as Keen Sword 2011, in the Pacific Ocean Dec 5, 2010. US and Japanese forces began military manoeuvres last Friday.[Photo/Agencies]

MOSCOW - Russian airplanes flew in the region of a joint US-Japanese military drill, a naval spokesman said on Wednesday, heightening tensions in a territorial dispute between Tokyo and Moscow.

"The area is our zone of responsibility. The airplanes carried out a planned flight in an area of the Russian Pacific Fleet's regular activity," said Roman Markov, a spokesman for the fleet.

"Our pilots did not violate any rules of international air space," he said.

Japan's Sankei newspaper had earlier reported that Russian patrol planes had interrupted joint US and Japanese military drills this week when they entered airspace above where the exercises were being conducted in the Sea of Japan.

Japan Maritime Self Defence Force destroyer Ikazuchi (L) sails alongside the USS George Washington during their military manoeuvres known as Keen Sword 2011, in the Pacific Ocean Dec 5, 2010. US and Japanese forces began military manoeuvres last Friday.[Photo/Agencies]

Markov said that two Ilyushin-38 airplanes -- a maritime patrol and an anti-submarine aircraft -- had made the flights.

Ties between Japan and Russia have been strained since Russian President Dmitry Medvedev infuriated Tokyo last month by becoming the first Russian leader to visit one of a string of islands claimed by both countries.

His trip to the islands -- called the Southern Kuriles in Russia and the Northern Territories in Japan -- caused Tokyo to recall its ambassador for consultations.

The Soviet Union occupied the four islands at the end of the war and the territorial row has weighed on ties between Tokyo and Moscow ever since, precluding a formal peace treaty.

Japanese Foreign Minister Seiji Maehara viewed the islands from a plane on Saturday, and media reports speculated that the Russian action could have been in response to the move.

Russia responded on Saturday by saying any country was free to view "Russia's natural beauty".

Politicians in both Japan and Russia have used tough talk on the dispute to bolster their credentials as patriots, but a Russian military source on Wednesday said the flights were not meant as a provocation.

"Similar flights of the Pacific Ocean Fleet occur regularly in the region and flights on Tuesday were carried out in the framework of military training," state-news agency Interfax quoted the high-ranking military source as saying.

Military exercises between the United States and Japan are taking place from December 3-10, with the participation of some 44,500 personnel.

The exercises are a regular occurrence, but this year come amid heightened tensions in the region after North Korea attacked a South Korean island and Seoul promised retaliation against any further attacks.

The row between Russia and Japan has spelled more bad news for Japanese Prime Minister Naoto Kan, whose support ratings sank this year, partly due to his handling of a spat with China over another set of islands that both nations claim.