Japan says two Russian fighter jets have violated its airspace, just hours after Prime Minister Shinzo Abe said he favored a "mutually acceptable solution" to a territorial dispute with Russia.
Japan's Defense Ministry said Tokyo scrambled its own planes in response after the intruders were detected off the coast of northernmost Hokkaido island for about one minute on February 7.
The Foreign Ministry also lodged a formal protest with Moscow.
A Russian military spokesman, Aleksandr Gordeyev, has denied the Japanese claim in a statement to Russian news agency Interfax.
Earlier on February 7, Japanese Prime Minister Abe said he was ready to do everything "toward sealing a peace treaty with Russia after resolving the issue of the Northern Territories."
The Northern Territories is the Japanese name for the Southern Kuriles, a group of islands seized by the Soviet Union from Japan in the last days of World War II.
After occupying the islands, Moscow expelled its Japanese population and brought in Russian citizens.
Russia and Japan have not signed a peace treaty formally ending World War II hostilities.
Speaking before some 2,000 former islanders and their descendants in Tokyo, Abe expressed hope that such a treaty could finally be achieved.
"We are continuing negotiations with the fundamental aim of settling the ownership issue of the four northern islands and concluding a peace treaty with Russia," Abe said.
"On that basis, I have strong hopes that we will make headway in our relationship with Russia and finally settle the Northern Territories dispute."
Abe said he had expressed his position in telephone talks with Russian President Vladimir Putin in December.
Abe's tone was in striking contrast to his uncompromising stance on a dispute with China over the sovereignty of a different set of disputed islands in the East China Sea, known as the Senkaku Islands in Japan and the Diaoyus in China.
With reporting by AP, AFP, dpa, Reuters, and Interfax