Russia's FSB security agency said on Monday it had detained a Japanese consul in Russia's Pacific port city of Vladivostok for suspected espionage and ordered him to leave the country.
The consul was released after a few hours of detention by the Russian agency. Tokyo has lodged a "strong protest" about the detention and signalled it may retaliate, Japan's Chief Cabinet Secretary Hirokazu Matsuno told a media briefing on Tuesday.
The FSB said the consul, Motoki Tatsunori, was declared persona non grata after he was caught "red-handed" receiving secret information on the effects of Western sanctions on the economic situation in Russia's far east.
It said the classified information, which also concerned Russia's cooperation with an unnamed Asia-Pacific country, had been obtained in return for a "monetary reward".
Moscow has protested to Tokyo over the consul's actions through diplomatic channels, it said.
Japan's top government spokesperson Matsuno said Moscow took the consul into custody in an "intimidating manner" blindfolding and restraining him, which was "a clear violation of the Vienna Convention on Diplomatic Relations".
"The detained consul was not engaging in any illegal activity," Matsuno said, adding the Japanese deputy foreign minister told the Russian ambassador to Japan that Tokyo "needs to take equivalent steps" and demanded a formal apology from Moscow.
The released consul has had no problem with health conditions and will depart Russia by Wednesday, Matsuno said.