"We have to take these threats seriously and we do... We've been monitoring, as best we can, his nuclear capabilities, I can tell you that we don't see any indication that we need to change our strategic deterrent posture at this point."
He dismissed plans for Russia to annex further parts of Ukraine as "nothing more than a ploy by Vladimir Putin to try to gain... through politics and electoral issues, that which he cannot gain militarily".
"But it's not going to work," he said. "No one's going to recognise it. And what needs to happen is Mr Putin needs to leave Ukraine. He needs to stop this war."
"This week, President Putin said Russia wouldn't hesitate to use 'all weapon systems available' in response to a threat to its territorial integrity - a threat all the more menacing given Russia's intention to annex large swaths of Ukraine in the days ahead..." said US Secretary of State Anthony Blinken.
"This from a country that in January of this year joined the other permanent members of the Security Council in signing a statement affirming that 'nuclear war can never be won and must never be fought'."
"This ignores the fact that for over eight years the Ukrainian army and fighters from the nationalist formations killed and continue to kill inhabitants of [the east Ukrainian region of] Donbas with impunity simply because they refused to recognise the results of the coup d'etat in Kyiv. They decided to uphold their rights, which were guaranteed by the Ukrainian Constitution, including the right to freely use Russian, their mother tongue."
In his speech on Wednesday, President Putin also announced a call-up for reservists in a move analysts say is a sign that Russia's forces in Ukraine are struggling to hold on to the strip of territory they occupy in the east and south.