Russia said it had protested to the Japanese embassy on Wednesday over joint Japan-U.S. military exercises this week in which it said HIMARS rocket systems were fired close to Russia's borders.
"We consider the military exercises that took place as a challenge to ensuring the security of the Far Eastern region of our country and insist on the immediate cessation of such actions," the Russian foreign ministry said in a statement.
"The Japanese side was also warned about the inevitability of adequate response measures in order to block military threats to Russia," it added, without elaborating.
HIMARS are the same rocket systems that the United States has supplied to Ukraine, which Kyiv has put to effective use in attacking Russian command nodes and supply lines.
Japan has joined Western countries in imposing sanctions on Russia in response to its Feb. 24 invasion of Ukraine. Relations have further worsened in recent weeks, with Russia expelling a Japanese consul for alleged espionage and Tokyo responding in kind.
(Reporting by Reuters; Editing by Andrew Cawthorne)
