Russian President Vladimir Putin has ordered a troop surge in Ukraine and is making pointed threats about nuclear retaliation. If his actions were meant to intimidate, they don't appear to be working. (Gavriil Grigorov/The Associated Press)

It's been called the nuclear blackmail card — both within western diplomatic and defence circles and in Russia itself.

And it's a reference to President Vladimir Putin's poker-faced threat on Wednesday to resort to weapons of mass destruction if NATO steps over the line or Ukraine reclaims more of its own occupied territory.

Insisting that "this is not a bluff," Putin warned that he has many such weapons at his disposal.

What might not be apparent from all the screaming headlines that followed — and from Putin's order to partially mobilize his country's military — is that his bluff is already being called in some respects. Ukraine believes he's playing with a hand that has grown increasingly weak.

The thinking in the Kremlin, according to a number of experts, is that Moscow's call for four referendums in the Ukrainian provinces its troops now occupy but do not fully control — Luhansk, Donetsk, Zaporizhzhia and Kherson — would formally draw them under Mother Russia's nuclear security dome.