Russian President Vladimir Putin will face "severe consequences" if he uses nuclear weapons in the Ukraine conflict, Downing Street warned.
Defence Secretary Ben Wallace has been in Washington for talks with his US counterpart amid reports Mr Putin could detonate a nuclear warhead over the Black Sea.
The Prime Minister's official spokesman did not comment on Mr Wallace's meeting but said: "We are very clear with Putin that the use of nuclear weapons will lead to severe consequences."
"We are taking a strong lead in this area"
The spokesman added: "I would guide away from speculating on this as an issue. I think the public need to be reassured that we are taking a strong lead in this area.
"I think it would be a mistake to be drawn into speculation on this rather than focusing on what we are seeing day by day, which is a senseless and barbaric attack on civilians across Ukraine."
It comes after The Sun newspaper quoted a security source who said the "threat" of Mr Putin's nuclear weapons "has increased recently".
Defence Secretary Ben Wallace earlier this month said that Vladimir Putin is "highly unlikely" to use nuclear weapons in the Ukraine conflict, but he is not acting in a "rational" way.
He said Mr Putin "was given a very clear sense what is acceptable and unacceptable" in meetings with the Indian and Chinese leaderships.
However, Mr Wallace added that the Russian leader's actions, from the nerve agent attack in Salisbury to the invasion of Ukraine, were "totally irrational".
The Defence Secretary recently pulled out of a planned session with the Commons Defence Committee on Tuesday to take part in talks with US counterpart Lloyd Austin.
A senior defence source said: "There has been a lot of speculation about why the Defence Secretary travelled to Washington.
"As we approach winter in Ukraine, with their successes on the battleground against Russian forces and in light of Putin's recent actions, it was important to meet face to face with US Secretary of Defence Lloyd Austin and others from the US Administration to discuss our shared security concerns."
On Wednesday, the Russian President declared martial law in the annexed regions of Ukraine, stepping up Kremlin control.