Russian Foreign Ministry spokeswoman Maria Zakharova said on Thursday that Russia could strike British military targets inside and outside Ukraine in response to Ukrainian strikes on Russia using British missiles.
Zakharova called on Britain to back off what she called London's hostile stance on Ukraine, which she said risked taking the conflict to an entirely new level.
She had told the same Moscow news briefing earlier that Britain and France were "playing with fire" by supplying sensitive missile technology to Ukraine.
Britain announced on Monday it would allow Kyiv access to classified technology to start its own production of Storm Shadow long-range cruise missiles, capable of carrying out strikes deep within Russia. France has already agreed to license the technology for the European missile.
Kremlin dismisses US media reports that Russia may attack NATO as 'scare-stories'
The Kremlin dismissed on Thursday US media reports that Russia could attack a NATO member state and that the purpose of CIA Director John Ratcliffe's visit to Moscow this week had been to warn Moscow against such a move.
Asked to comment on the reports, Kremlin spokesman Dmitry Peskov said: "A lot of this kind of scare-story material is being published right now. Of course, it has nothing to do with reality and nothing to do with the intentions of the Russian Federation."
Earlier on Thursday, the head of Russia's SVR foreign intelligence service said he held a "working-level meeting" with Ratcliffe and that they discussed unspecified, sensitive matters within the remit of their respective intelligence services.