From NPR (in a piece entitled "Putin warns of nuclear war if NATO sends troops to Ukraine):
"Russian President Vladimir Putin delivered his annual state of the nation address on Thursday, issuing explicit nuclear threats to the West...
"Putin warned that if Ukraine's Western backers deepened their involvement in the war, such as sending troops, the consequences for the invaders" would be 'tragic' and it would risk starting a nuclear war."
(I very much doubt that he is serious--this is almost certainly just bluster--but it helps to demonstrate just how desperate he is now getting.)
Here is the link: Ukraine invasion — explained : NPR