(Reuters) - Russia on Saturday accused Japan of "cynical, unscrupulous speculation" over Tokyo's comments around the nuclear threat Moscow poses and promised to respond to Japan's latest round of sanctions imposed over Russia's invasion of Ukraine.

Chief Cabinet Secretary Hirokazu Matsuno on Friday said Japan would place additional sanctions on Russia after the Group of Seven (G7) summit Tokyo hosted last week agreed to step up measures to punish Moscow's invasion of Ukraine.

Matsuno also condemned Russia's plan to deploy tactical nuclear weapons in Belarus, saying it would further inflame the situation and that Japan would never accept "Russia's nuclear menace, let alone its use".

Russia's foreign ministry said it was assessing the implementation of Japan's sanctions on its national security and economy and would not leave Tokyo's "illegitimate actions" unanswered.

The ministry also took issue with Matsuno's casting of Russia as engaging in "nuclear blackmail".

"The desire to attribute the non-evident intention to use nuclear weapons in relation to events in Ukraine to Russia is nothing more than cynical, unscrupulous speculation," the ministry said.

Russia earlier on Saturday dismissed criticism from U.S. President Joe Biden over Moscow's plan to deploy tactical nuclear weapons in Belarus, saying Washington had for decades deployed such nuclear weapons in Europe.

The foreign ministry urged Japan to show similar "concern" towards U.S. deployments in Europe.

(Reporting by Alexander Marrow; Editing by Conor Humphries)