(NewsNation) — Russian President Vladimir Putin’s decision to mobilize 300,000 reserve troops and his subsequent veiled threat about using nuclear weapons in Ukraine were made because he is “feeling the pressure” of a losing war, said National Security Council spokesperson John Kirby.

After suffering shocking battlefield losses in parts of northern and eastern Ukraine, Western officials believe Putin is now facing “pressure at home” and “pressure on the battlefield” to turn the tide of an invasion into Ukraine that has not gone to plan for Russia, Kirby said in a Wednesday interview on NewsNation’s “Dan Abrams Live.”

Putin further escalated global tensions when he said Russia was “not bluffing” about using all weapons at their disposal in yet another threat to use nuclear weapons if things do not go his way in Ukraine. Kirby said despite this type of rhetoric from Putin being nothing new, the U.S. is still monitoring the potential use of nuclear weapons closely.

“These are the kinds of words he’s used before; he has been talking threatening about nuclear weapons since the beginning of the conflict,” Kirby said. “That doesn’t mean we don’t take it seriously and it doesn’t mean we aren’t monitoring it. We are. We just haven’t seen any indication we need to change our strategic deterrent posture.”

Kirby also reiterated the United States is ready to respond to whatever Russia decides to do if needed, noting the 20,000 troops the U.S. moved to eastern Europe at the onset of Putin’s war, saying U.S. readiness is “very high.”

“One of the things we’ve been concerned about since the beginning was the Putin reaction and the potential for escalation of this conflict,” Kirby said. “As we’ve long said, escalating this conflict beyond what it is now, certainly in the realm of weapons of mass destruction, that’s not good not only for the Ukrainian people, it’s not good for Europe, it’s not good for our national security interests.”

Pressure on Putin has been mounting since the Ukrainian military launched an offensive in northeastern Ukraine, which saw Russia sustain heavy losses and retreat quickly as they were overwhelmed by Ukrainian forces in the region near Kharkiv in cities such as Izium.

Putin has now launched a plan to have the Russian-occupied parts of eastern Ukraine vote on a referendum to rejoin Russia, a vote the United States has called a “sham.”

U.S. President Joe Biden on Wednesday assailed Putin at the United Nations General Assembly for his invasion of Ukraine and said Putin has “shamelessly violated the core tenets of the U.N. charter.”

Putin, on the other hand, has continued to lambast the United States and other NATO countries for aiding Ukraine’s defense effort, claiming the west is encroaching on Russian interests. Putin has painted his war as one in which he is “liberating” Ukrainian people from an “oppressive” regime he claims is run by Nazis despite the nation’s president being Jewish.

The United States has held true on its position to support Volodymyr Zelenskyy as the president of Ukraine and, according to Kirby, says it will be up to Zelenskyy to lead his nation’s war effort — with the backing of U.S. weapons, of course.

“Our position is President Zelenskyy gets to determine how and when the war ends and under what circumstances,” Kirby said. “He gets to determine success or failure and he’s done that. He’s been very clear about what his war aims and objectives are. We respect that. He’s the commander-in-chief.”