Nato is ready for a "direct confrontation" with Russia, a top admiral has warned.
Rob Bauer, chair of the Nato Military Committee, said "we are ready” when asked if the US-led organisation was prepared for a confrontation with Vladimir Putin's regime amid the war in Ukraine.
But he stressed he did not think there would be a nuclear conflict between the two military powers as Putin was "not insane" and was still a "rational" person.
Bauer also told Portugal's RTP TV channel that Nato would only respond if Russia crossed a red line by invading a Nato member state.
Bauer said Nato had started moving battle groups along the eastern front and had created four more battle groups following Russia's invasion of Ukraine.
He said this was an important message for Moscow to show Nato's posture had changed.
It comes after Ukraine secured pledges from the West in January to supply main battle tanks to help fend off Russia's full-scale invasion.
Watch: Putin made big strategic mistake in underestimating us, says NATO General Secretary Jens Stoltenberg
Ukraine will receive 120 to 140 Western tanks in a first wave of deliveries from a coalition of 12 countries, including the UK and US.
Nato member Germany was also part of the coalition after finally agreeing to send Leopard tanks last week.
Since winning the pledge for tanks, Kyiv has pressed on with further requests for arms, including calls for jet fighters such as US F-16s.
But the West has so far refused to send weapons that could be used to attack deep inside Russia and US president Joe Biden responded with a flat "no" when asked by reporters if Washington would send F-16s.
Rishi Sunak's spokesperson said on Tuesday that London did not believe its own jets would be useful, and given that learning to fly them would take months, it would not be practical to send them to Ukraine.
On Sunday, German chancellor Olaf Scholz also confirmed he would not be sending fighter jets to Ukraine.
He warned against escalating with Moscow, telling the Tagesspiegel newspaper: "There is no war between Nato and Russia."
Scholz added he would continue to talk directly to Putin.
Front lines in eastern Ukraine have largely been frozen since November after Kyiv recaptured swathes of territory in the second half of last year.
But momentum has lately swung back towards Russia as it has made incremental gains for the first time since mid-2022.
Military experts say Moscow appears determined to push forward in the coming months before Kyiv receives hundreds of newly-pledged Western battle tanks and armoured vehicles for a counter attack to recapture occupied territory this year.
Kyiv says the Russian assaults of recent weeks have come at a huge cost, initially mostly relying on Wagner mercenaries, including thousands of convicts recruited from Russian prisons and sent into battle in waves with little training or equipment.
But Russia's call-up of hundreds of thousands of reservists late last year means Moscow has now been able to reconstitute regular military units exhausted or depleted earlier in the war.
Russia claimed on Tuesday to have captured a village just to the north of Bakhmut, a city it is trying to surround in a major push for what would be its biggest battlefield win in Ukraine since last summer.