Russia said on Wednesday that if the United States stationed missiles in Japan, this would threaten Russian security and prompt Moscow to retaliate.
Japan's Kyodo news agency reported on Sunday that Japan and the U.S. aim to compile a joint military plan for a possible Taiwan emergency that includes deploying missiles.
It cited unnamed U.S. and Japanese sources as saying that under the plan, the U.S. would deploy missile units to the Nansei Islands of Japan's southwestern Kagoshima and Okinawa prefectures, and to the Philippines.
Russian Foreign Ministry spokeswoman Maria Zakharova accused Japan of escalating the situation around Taiwan to justify the expansion of military ties with Washington.
"We have repeatedly warned the Japanese side that if, as a result of such cooperation, American medium-range missiles appear on its territory, this will pose a real threat to the security of our country and we will be forced to take the necessary, adequate steps to strengthen our own defense capability," she said.
Zakharova said Tokyo could get an idea of what such steps would entail by reading Russia's updated nuclear doctrine, published last week, which expanded the list of scenarios under which it would consider using nuclear weapons.
On Monday, Deputy Foreign Minister Sergei Ryabkov said Russia would consider deploying short- and intermediate-range missiles in Asia if the United States deployed such missiles to the continent.
Asked about that statement, Zakharova declined to discuss where Russia might site such weapons, but noted that half its territory is in Asia so any Russian missiles potentially deployed east of the Urals would be in that region.
She said Moscow had sent a clear signal to the United States and its "satellites" that Russia would respond decisively and in symmetrical fashion to the placing of land-based medium and shorter-range missiles in various parts of the world.
She said the West should have no doubts about Russia's potential after it launched a new hypersonic intermediate-range missile, the Oreshnik, at a target in Ukraine last week.