Ben Wallace dashed from London to Washington for urgent talks with officials in the US amid growing fear about Putin’s next move.
The Russian leader has made a series of nuclear threats which have intensified in recent weeks, with speculation he could detonate a nuclear bomb over the Black Sea as a warning shot to NATO.
Prime Minister Liz Truss warned the UK would come down hard on any use of nuclear weapons.
The Prime Minister’s official spokesman said: “We are very clear with Putin that the use of nuclear weapons will lead to severe consequences.”
“I think the public need to be reassured that we are taking a strong lead in this area.”
Meanwhile, US President Joe Biden has claimed the world is closer to nuclear war today than it has been at any point since The Cuban Missile Crisis in 1962.
Ukrainian gains have forced Putin to up his threats in recent months: attempting to draft in hundreds of thousands of extra troops, illegally annexing four Ukrainian regions and threatening to resort to nuclear weapons to defend what Russia regards as its own land.
Kremlin spokesperson Dmitry Peskov told reporters on Monday Russia could use nuclear weapons to defend newly-annexed areas in eastern Ukraine.
On Wednesday, Putin declared martial law in the annexed regions, stepping up Kremlin control.
Almost a third of Ukraine’s power plants had been destroyed since October 10, Ukrainian President Volodymyr Zelensky said on Wednesday.
Writing on Twitter Mr Zelensky blasted “another kind of Russian terrorist attack” after missiles hit plants, relay stations and waterworks across the country, including in Kyiv.
The strikes on power stations appear to signal a new tactic from Russia attempting to deprive Ukrainians of water, electricity and heating.
Mr Zelenskiy met with senior officials on Wednesday to discuss security at power stations and vowed “Ukraine will defend itself”.
He said they discussed measures to “eliminate the consequences in the event of a breakdown of the energy system of Ukraine”.
He wrote on the Telegram messaging app: “We are working to create mobile power points for the critical infrastructure of cities, towns and villages.”
In the last week over 100 drones have struck power and sewage plants, residential buildings, bridges as well as other targets according to the Ukrainian foreign ministry.
On Monday, five people died in Kyiv when a building across the road from a heating plant was struck by a drone.
Two more were killed during a strike on a power station in western Kyiv.