Russian Foreign Minister Sergei Lavrov has warned that any future world war 'will become nuclear'.
Lavrov defended the invasion of Ukraine by claiming Russia would face a "real danger" if Kyiv acquired nuclear weapons.
It comes after UK Defence Secretary Ben Wallace ruled out imposing a no-fly zone over Ukraine, warning it could spark a new world war.
Meanwhile Ukrainian troops continue to fiercely resist Russian advances on the ground, although the city of Kherson has reportedly been captured by the invaders.
Mr Wallace warned that Russia would use indiscriminate carpet bombing tactics against Ukrainian cities as their invasion drags on.
He told Sky News that Russian advances were being hampered by resistance they had seen "footage of Ukrainians using UAVs to attack petrol train convoys, to go after logistical lines, we have seen railway lines blown up" but there had also been a switch in Moscow's tactics as a result.
"What you are seeing now is those heavy bombardments at night, they won't come into the cities as much, they will - I'm afraid, as we have seen tragically by the looks of things - carpet-bomb cities, indiscriminately in some cases."
However, he ruled out imposing a no-fly zone over Ukraine.
He told GMB: "I think, first of all, to put in a no-fly zone, you have to enforce it because the Russians wouldn't comply with it voluntarily.
"And that would mean British, French German fighter jets enforcing it by shooting down Russian aircraft. They would retaliate they've got lots of anti-air missiles, etcetera.
"That would trigger a NATO article five and under NATO those countries in NATO that's 30 nations will come to the aid of any member who triggers this Article 5 self-defence clause.
"So very rapidly, we would go from a war in Ukraine to a war right across Europe, and indeed the United States and Canada as members of NATO, and a war against Russia as a state."
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Meanwhile, Ukrainian President Volodymyr Zelensky said 6,000 Russian troops had been killed during the first six days of the war.
The city of Kherson has been surrounded by Russian troops and the Kremlin claims it has seized control.
Videos shared on social media purport to show tanks rolling through Kherson's streets - although the Ukrainian Government has not yet confirmed it has lost the city.
Russian paratroopers dropped into the city of Kharkiv overnight, following a brutal bombardment throughout the day.
At least 21 people were killed by shelling in the city overnight, with 112 injured.
