Russian President Vladimir Putin ordered his military command to put nuclear-armed forces on high alert as Ukrainian fighters defending the city of Kharkiv said they had repelled an attack by invading Russian troops.
US Ambassador to the United Nations Linda Thomas-Greenfield said that "President Putin is continuing to escalate this war in a manner that is totally unacceptable and we have to continue to stem his actions in the strongest possible way".
On the fourth day of the biggest assault on a European state since World War Two, the Ukrainian president's office said negotiations between Kyiv and Moscow would be held at the Belarusian-Ukrainian border. They would meet without preconditions, it said.
Ukrainian soldiers stand past a burnt Ukrainian army vehicle on the west side of the Ukrainian capital of Kyiv. Source: AFP / DANIEL LEAL/AFP via Getty Images
The capital Kyiv was still in Ukrainian government hands, with President Volodymyr Zelenskyy rallying his people despite Russian shelling of civilian infrastructure.
But Mr Putin, who has described the invasion as a "special military operation", thrust an alarming new element into play on Sunday when he ordered Russia's deterrence forces - a reference to units which include nuclear arms - onto high alert.
He cited aggressive statements by NATO leaders and economic sanctions imposed by the West against Moscow.
"As you can see, not only do Western countries take unfriendly measures against our country in the economic dimension - I mean the illegal sanctions that everyone knows about very well - but also the top officials of leading NATO countries allow themselves to make aggressive statements with regards to our country," Mr Putin said on state television.
Mr Putin's order is part of a pattern of Moscow manufacturing threats to justify aggression, White House Press Secretary Jen Psaki said on Sunday.
"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" 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," Ms Psaki added.
Mr Putin's decision to place his nuclear forces on alert amounts to "irresponsible" behavior, NATO charged Sunday.
"This is dangerous rhetoric. This is a behavior which is irresponsible," the alliance's secretary-general Jens Stoltenberg told CNN amid heightened tensions over the Russian invasion of Ukraine.
"And, of course, if you combine this rhetoric with what they're doing on the ground in Ukraine, waging war against the independent sovereign nation, conducting a full-fledged invasion of Ukraine, this adds to the seriousness of the situation,” he added.
'Control over Kharkiv is completely ours'
Russian soldiers and armoured vehicles have rolled into Kharkiv, Ukraine's second-largest city, and witnesses reported firing and explosions. A natural gas pipeline was blown up before daybreak, a Ukrainian state agency said.
But city authorities said Ukrainian fighters had repelled the attack.
"Control over Kharkiv is completely ours! The armed forces, the police, and the defence forces are working, and the city is being completely cleansed of the enemy," regional Governor Oleh Sinegubov said.
Reuters was unable to immediately corroborate the information.
Ukrainian forces were also holding off Russian troops advancing on Kyiv.
"We have withstood and are successfully repelling enemy attacks. The fighting goes on," Mr Zelenskyy said in a video message from the streets of Kyiv.
