The United States and NATO have condemned Russian President Vladimir Putin's order to put his nuclear forces on high alert as dangerous and unacceptable.
The White House said it has not ruled out imposing new sanctions on Russia's energy sector.
In issuing the order to prepare Russia's nuclear weapons for increased readiness for launch, Mr Putin cited "aggressive statements" from NATO allies and widespread sanctions imposed by Western nations that have already disrupted his country's economy.
The US ambassador to the United Nations, Linda Thomas-Greenfield, said on CBS's Face the Nation programme that Mr Putin's actions have escalated the conflict and were "unacceptable".
Thomas-Greenfield said the United States welcomed the news that Russian and Ukrainian officials would meet for talks on the border with Belarus, but that it "remains to be seen" if Russia is acting in good faith.
Asked if there is a threat of chemical and biological weapons being used by Russia, Thomas-Greenfield said of Mr Putin: "Certainly nothing is off the table with this guy. He's willing to use whatever tools he can to intimidate Ukrainians and the world."
At the Pentagon, a senior US defence official called Mr Putin's nuclear order escalatory and said it was "putting in play forces that, if there's a miscalculation, could make things much, much more dangerous".
The United States is trying to determine what Mr Putin's order means "in tangible terms," said the defence official, speaking on condition of anonymity.
Asked whether the United States would continue to provide military assistance to Ukraine following Mr Putin's announcement, the defence official said: "That support is going to go forward."
Citing a Russian offensive on the Ukrainian city of Chernihiv, north of Kyiv, the defence official cited early indications that Russia was adopting siege tactics.
"It appears that they are adopting a siege mentality, which any student of military tactics and strategy will tell you, when you adopt siege tactics, it increases the likelihood of collateral damage," the defence official said.
On CNN's State of the Union programme, NATO Secretary-General Jens Stoltenberg called Mr Putin's behaviour "aggressive" and "irresponsible".
White House Press Secretary Jen Psaki said Mr Putin, in his nuclear order move, was responding to an imaginary threat.
"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," Ms Psaki said on ABC's This Week programme.
"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," Ms Psaki added.
The United States has not taken sanctions targeting Russia's energy sector off the table, Ms Psaki said.
"We want to take every step to maximize the impact of consequences on President Putin while minimising the impact on the American people and the global community. And so energy sanctions are certainly on the table. We have not taken those off," Ms Psaki said.
"But we also want to do that and make sure we're minimising the impact on the global marketplace and do it in a united way," Ms Psaki added.
The Biden administration has been concerned that its sanctions could raise already-high gas and energy prices in the United States and has taken steps to mitigate that.
When it issued sanctions targeting major Russian banks on Thursday, it authorised energy-related transactions to continue.
The United States is open to providing additional assistance to Ukraine, Ms Psaki said.
US Secretary of State Antony Blinken today announced $54 million in new humanitarian aid for Ukrainians affected by the invasion, which was in addition to the $350 million sent by the United States last week.
"This includes the provision of food, safe drinking water, shelter, emergency health care, winterization, and protection," Mr Blinken said in a statement.
UN nuclear watchdog to meet over fallout from invasion
The UN nuclear watchdog has called a meeting of its governing body to discuss the fallout from Russia's invasion of Ukraine, days after Moscow's troops took control of the Chernobyl site.
The International Atomic Energy Agency (IAEA) said in a statement that the meeting of its board of governors would be held Wednesday at its Vienna headquarters.
Ukraine has four active nuclear power plants, as well as stores of nuclear waste such as the one at Chernobyl, and there have been fears over the possible consequences should they be damaged in the fighting.
Chernobyl was the site of the worst nuclear accident in history in 1986 and on Thursday the site fell to Russian troops.
Ukrainian authorities said that radiation levels had increased there following the Russian takeover but the IAEA said the levels remained low and did not "pose any danger to the public".
Also today the IAEA said it had been informed by Ukraine that "missiles hit the site of a radioactive waste disposal facility in Kyiv overnight but there were no reports of damage to the building or any indications of a radioactive release".
That incident came a day after Ukrainian officials said an electrical transformer at a similar disposal facility near the northeastern city of Kharkiv had been damaged, but again there had been no reports of a radioactive release.
IAEA Director General Rafael Grossi said such incidents "highlight the very real risk that facilities with radioactive material will suffer damage during the conflict, with potentially severe consequences for human health and the environment".
"Once again, I urgently and strongly appeal to all parties to refrain from any military or other action that could threaten the safety and security of these facilities," he added.
The IAEA's meeting on Wednesday will come days before a scheduled board of governors meeting and coincides with a crucial phase of efforts to fully restore the 2015 deal on Iran's nuclear programme.
Diplomats from Britain, China, France, Germany, Iran and Russia have been taking part in talks on reviving the accord in Vienna, with their success or failure widely expected to hinge on negotiations in the coming days.
The United States has been taking part in the talks indirectly.
