In a Thursday statement Russian Deputy Foreign Minister Sergei Ryabkov said "Moves in this direction will not provide greater security (for Poland or other nations that host such weapons)." He was further quoted in Russian media as saying: 

Moscow considers any expansion of NATO's nuclear-sharing arrangement as "deeply destabilizing" in nature, "and in fact threatening" Russia, Ryabkov was quoted as saying by TASS on Thursday. This applies to joint missions, where non-nuclear members of the US-led bloc are trained to use American hardware, and even more so to the permanent stationing of such weapons "which hotheads in Warsaw are talking about," he said.

In follow-up, Foreign Ministry spokeswoman Maria Zakharova stipulated that "any nuclear weapons deployed to Poland would be legitimate targets in the event of war with the alliance." Russian media translations said Moscow would see this as a "priority target".

All of this was in response to words issued by Polish President Andrzej Duda in a Monday interview. He said his country is "ready" to host nuclear weapons should NATO decide to do so as reinforcement of its eastern flank. It remains that the United States chiefly supplies and oversees NATO's nuclear-sharing program, thus any possible future nukes in Poland would be supplied by Washington.

"Russia is increasingly militarizing the Königsberg oblast (Kaliningrad). Recently, it has been relocating its nuclear weapons to Belarus," Duda continued, apparently wanting to match and mirror Russian moves. Indeed Belarus is now believed to host possibly dozens of Russian tactical nukes.