White House spokesperson Karine Jean-Pierre reiterates the US' stance which affirms that the United States takes Russian President Vladimir Putin's words about nuclear weapons use seriously but sees no need to boost the readiness of its own strategic deterrent forces at the moment.

"We obviously take these [Russian nuclear] threats very seriously, but we have not seen any reason to adjust our own nuclear posture at this time, “she said.

Jean-Pierre also claimed that the US does not seek conflict with Russia and is concerned about escalation in the Ukraine crisis.

"We do not seek conflict with Russia. Of course, we are attentive to managing escalation in this conflict as always, as we have been for the past six months or more of this war, and throughout this year," Jean-Pierre told reporters.

National Security Coordinator John Kirby said, on Wednesday, "We always have to take this kind of rhetoric seriously. It is irresponsible rhetoric for nuclear power to talk that way."

The US official added that the US is monitoring the situation but sees no need to boost its own strategic forces' readiness.

The remarks came after Putin signed an order calling up 300,000 reservists to support Russian forces in Ukraine.

The Russian President also accused the west of using the threat of nuclear weapons to blackmail Russia and warned that Moscow will use all means in case its territorial integrity is threatened.

Read more: Putin: No winners in nuclear war