On Monday, an ally of Ms Truss hit back after the Kremlin’s claim circulated, suggesting it was being made because Russia was not having the military success it hoped for in Ukraine.
A Foreign Office source said: “It’s a blatant attempt to distract from the slowing invasion. Nothing Liz has said warrants that sort of escalation.
“She’s clear Nato is a defensive alliance, and that we’re providing defensive support to Ukraine only.
“Liz has helped lead a tough international response on sanctions and support for Ukraine, and that has clearly got under the skin of the Kremlin.”
Support for Ms Truss’s position came from Nicola Sturgeon, the Scottish First Minister, who as the SNP’s leader is usually politically opposed to the Conservatives.
Ms Sturgeon said: “Whatever political disagreements any of us have with Liz Truss – and I have many deep differences with her – we should not fall for this transparent Russian attempt to divert.
“The only person responsible for Putin’s despicable nuclear threat is Putin.”
UK and US government figures moved on Monday to calm concerns over Mr Putin’s declaration, noting that the phrase “special mode of combat service” was not a known part of Russia’s nuclear doctrine.
‘It’s not a term of art’
Ben Wallace, the Defence Secretary, said his 12-year-old son had called him worried about the step, as he offered reassurance over the significance of the order.
“We don’t see or recognise in the sort of phrase or the status he described as anything that is a change to what they have currently as their nuclear posture,” Mr Wallace told BBC Breakfast.
“This is predominantly about Putin putting it on the table just to remind people, remind the world, that he has a deterrent.
“We will not do anything to escalate in that area, we will not do anything to feed any miscalculation, we take it very, very seriously.
“But at the moment this is a battle of rhetoric that President Putin is deploying, and we just have to make sure we manage it properly.”
A US administration official quoted by Reuters said: “It’s not a term of art in what we understand to be Russian (nuclear) doctrine.
“So that’s why we’re analysing it and reviewing it to try to understand what exactly it means.”
The official added: “I don’t believe we’ve seen anything specific as a result of the direction that he gave, at least not yet, in terms of appreciable or noticeable muscle movements.”
