Russia has issued a chilling threat, saying it will launch nuclear weapons against the West if Nato forces enter Ukraine.

The disturbing warnings were made by commentators on President Vladimir Putin's highly controlled state TV on Wednesday, ahead of Thursday's emergency summit in Brussels between Nato, G7 and European Union leaders.

According to a report by news service East2West, several pro-Kremlin mouthpieces claimed Poland was seeking a Nato mandate to put peacekeepers in Ukraine.

"If there are any sane people left in Nato, they will not approve [a peacekeeping] operation [in Ukraine]," military expert Colonel Yury Knutov said on state-owned Channel 1.

"Why? Because [a collective] Nato decision will mean a de facto declaration of war on Russia. To win this war, whether we like it or not, we will have to use tactical nuclear weapons in the theatre of operations."

Colonel Knutov said this would entail the "use of powerful strategic nuclear weapons" which he said "means universal nuclear war".

Olga Skabeyeva, host of 60 Minutes on state-owned Rossiya 1, similarly said Nato peacekeepers entering Ukraine would be "called World War III".

And military veteran Colonel General Vladimir Shamanov, known as the "Butcher of Chechnya", called on Putin to secure the border between Ukraine and Poland.

"The time has come for our leadership to say very clearly, 'these are the borders,'" he told Channel 1.

"And don't you Poles dare poking in here, even 10m. You'll be hit immediately with the whole might of our Kalibr [cruise missiles]."

Kremlin spokesman Dmitry Peskov said earlier this week Russia would only use nuclear weapons in the face of an "existential threat".

But he warned deploying Western peacekeepers to Ukraine would be "very rash and extremely dangerous".

"Any possible contact between our military personnel and Nato military personnel could lead to quite understandable consequences that would be difficult to put right," he told CNN International.

It comes as a growing number of experts warn the Russian leader is being backed into a corner, raising the chances of nuclear weapons being deployed.

Former intelligence analyst Dr David Wright-Neville told 3AW on Thursday the Ukraine invasion was not going well and Putin was "lashing out".

Russia has suffered heavy losses in Ukraine. Western countries estimate 7000 to 14,000 are dead and another 30,000 to 40,000 have been injured or captured.

"We're getting to the point where it's roughly about 9 to 10 per cent," Dr Wright-Neville told 3AW host Neil Mitchell.

"That's that tipping point at which they can no longer really do the functions that they're set up to do, so he's resorting to missiles. He's shot about 1200 missiles over the last 28 days. Well you can shoot them faster than you can build them. At some point he's going to run out and he's going to lash out."

Wright-Neville said he was becoming "more and more worried" about the possibility of a nuclear attack.

"There's some suggestion emerging that there are Russian military documents from the 2010s that suggest a limited one-off nuclear strike to convey the message that Russia is not afraid of escalation would be one way of proceeding," he said.

But military experts predict any nuclear strike would provoke a retaliation, rapidly spiralling into all-out conflict.

"I suspect that there's intelligence that the Americans have and the Europeans have that points to the possibility that he's going to use chemical weapons before he pulls the nuclear trigger," Wright-Neville said.

"I think that would probably be the next step."

Nato Secretary General Jens Stoltenberg speaks during a media conference during an extraordinary Nato summit at Nato headquarters in Brussels, March 24, 2022

In a press conference following Thursday's meeting, President Joe Biden said the US and Nato would react if Russia used chemical weapons.

"We would respond if he uses it," he said. "The nature of the response would depend on the nature of use."

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Earlier, Nato Secretary General Jens Stoltenberg had said member states were preparing to provide Ukraine with equipment to "protect against chemical, biological, radiological and nuclear threats".

'Increasing the pain' on Moscow

As the war in Ukraine entered its fifth week, Western leaders including Biden, UK Prime Minister Boris Johnson and French President Emmanuel Macron gathered in Belgium to discuss overhauling the alliance's eastern defences.

Ukrainian President Volodymyr Zelensky pressed the Nato summit to flood weapons into Ukraine and Western allies responded with new sanctions against Russia, promises of military aid, and discussion of expelling Moscow from the international G20 body.

He said the West should provide "all the weapons we need" to "prevent the deaths of Ukrainians from Russian strikes, from Russian occupation".

Kicking off a day of intense diplomacy, Biden made clear that the Western alliance was listening.

"Nato has never been more united," he said.

After the US announced new sanctions, including targeting Russian politicians, Biden said the West was in it for the long haul, intent on "increasing the pain" on Moscow.

Zelensky has been asking Nato to help Ukraine go on the offensive with more advanced fighter jets, missile defence systems, tanks, armoured vehicles and anti-ship missiles.

Nato members have supplied a steady stream of weapons including antitank rockets, which have helped to stall Russia's advance. But these are seen as essentially defensive.

The US has so far ruled out sending aircraft or other large weapons systems to Ukraine. Biden says he does not want to cross a line into what he says could become "World War III" pitting nuclear-armed Russia against Nato.

However, Secretary General Stoltenberg announced new deployments to eastern flank members Romania, Hungary, Slovakia and Bulgaria, as well as bolstering chemical and nuclear defences in case Russia expands its attack beyond Ukraine.

Biden said Nato unity showed "Putin is getting exactly the opposite of what he intended".

Thousands of Ukrainian civilians, as well as thousands of soldiers from the two sides, are believed to have been killed since Russia invaded on February 24.

More than 10 million Ukrainians have fled their homes. And the month of war has displaced 4.3 million children – more than half of Ukraine's estimated child population of 7.5 million.

"This is a grim milestone that could have lasting consequences for generations to come," Unicef chief Catherine Russell said.

UN figures show that nearly 3.7 million Ukrainians have fled abroad, and more are now displaced inside Ukraine after harrowing journeys out of bombarded cities like Mariupol.