Russian President Vladimir Putin ordered his military command to put nuclear-armed forces on high alert on Sunday as Ukrainian fighters defending the city of Kharkiv said they had repelled an attack by invading Russian troops.

The United States responded by saying Putin was escalating the war in a "totally unacceptable" way.

Nato chief: Putin's nuclear rhetoric is 'dangerous' and irresponsible

Jens Stoltenberg, the secretary general of Nato, said “this is dangerous rhetoric” over Russian president Vladimir Putin’s order to put the country’s nuclear military deterrence on high alert.

“This is dangerous rhetoric. This is a behaviour which is irresponsible,” Stoltenberg said on CNN.

“And of course when you combine this rhetoric with what they are doing on the ground in Ukraine – waging war against an independent, sovereign nation, conducting full-fledged invasion of Ukraine – this adds to the seriousness of the situation.”

More to come as the US reaches mid-morning on the east coast and the broadcast round begins in earnest. US ambassador to the UN Linda Thomas-Greenfield is speaking to NBC.

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.

As missiles rained down on Ukrainian cities, thousands of Ukrainian civilians, mainly women and children, were fleeing from the Russian assault into neighbouring countries.

The capital Kyiv was still in Ukrainian government hands, with President Volodymyr Zelenskiy rallying his people despite Russian shelling of civilian infrastructure.

But 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," Putin said on state television.