Japan is concerned by the holding of Russia's Vostok-2010 military drills on Iturup Island, the largest of the disputed South Kuril Islands, Japan's Kyodo news agency said.

"This is extremely regretful. As far as we know, there have never been such drills there," Japanese Foreign Minister Katsuya Okada told reporters late on Tuesday.

The Russian Armed Forces started large-scale Vostok-2010 military exercises in Siberia and the country's Far East on June 29. The drills, which involve at least 20,000 troops, up to 70 warplanes and 30 warships, will continue through Thursday.

Okada said that Japan had earlier told Russia about its objections to the drills, but to no avail.

Tokyo's continued claim over four South Kuril Islands (Iturup, Kunashir, Shikotan, and Habomai) to the northeast of Japan has so far prevented Russia and Japan from signing a formal peace treaty to end World War II hostilities. The islands were annexed by the Soviet Union after World War II.

A senior source in the Russian Navy earlier said that the naval phase of the drills have drawn increased attention from the intelligence services of the United States and Asia-Pacific countries. Increased activity by intelligence aviation, particularly Japanese, has been noticed in close proximity to the area where the drills are being held, he said.

TOKYO, July 7 (RIA Novosti)