White House press secretary Jen Psaki said Sunday that Russian President Vladimir Putin's order to put nuclear forces on high alert is part of "a pattern of Moscow manufacturing threats to justify aggression."
"We've seen him do this time and time again. At no point has Russia been under threat from NATO, has Russia been under threat from Ukraine," Psaki said on ABC's "This Week" program.
"This is all a pattern from President Putin and we're going to stand up to it. We have the ability to defend ourselves, but we also need to call out what we're seeing here from President Putin," Psaki added.
The United States is open to providing additional assistance to Ukraine, Psaki said. Washington also has not taken sanctions targeting Russia's energy sector off the table, Psaki added.
"We have not taken those off, but we also want to do that and make sure we're minimizing the impact on the global marketplace and do it in a united way," Psaki said.
In a dramatic escalation of East-West tensions over Russia's invasion of Ukraine, Putin ordered Russian nuclear forces put on high alert Sunday in response to what he called "aggressive statements" by leading NATO powers.
The order means Putin has ordered Russia's nuclear weapons prepared for increased readiness to launch, raising the threat that the tensions could boil over into nuclear warfare. In giving it, the Russian leader also cited hard-hitting financial sanctions imposed by the West against Russia, including Putin himself.
Speaking at a meeting with his top officials, Putin directed the Russian defense minister and the chief of the military's General Staff to put the nuclear deterrent forces in a "special regime of combat duty."
The alarming step came as street fighting broke out in Ukraine's second-largest city and Russian troops squeezed strategic ports in the country's south, advances that appeared to mark a new phase of Russia's invasion following a wave of attacks on airfields and fuel facilities elsewhere in the country.
The U.S. ambassador to the United Nations responded to the news from Moscow while appearing on a Sunday news program.
"President Putin is continuing to escalate this war in a manner that is totally unacceptable," Ambassador Linda Thomas-Greenfield said. "And we have to continue to condemn his actions in the most strong, strongest possible way."
Ukranian Foreign Minister Dmytro Kuleba said Sunday that Ukraine will not surrender to Russia's invasion of the country as he accused Putin of seeking to increase "pressure" by ordering his nuclear forces on high alert.
"We will not surrender, we will not capitulate, we will not give up a single inch of our territory," Kuleba said at a news conference broadcast online.
Kuleba also said Russia dropped its preconditions for talks after suffering military setbacks, adding that Ukraine would attend the talks to listen to what Russia had to say.