Britain has warned Vladimir Putin that if a "single toecap" of a Russian solider steps into NATO territory "then it would be war".
Health Secretary Sajid Javid said the West had been clear over the response to any aggression on its territory following a bombing near the border with Poland.
The Yavoriv military base was targeted in a deadly barrage of missiles on Sunday, only a few miles from the Polish border.
People were woken by sirens before the “skies turned red” as missiles landed, killing 35 and leaving 134 wounded.
Mr Javid condemned the attack and warned Putin that he could face war with 30 NATO countries if he moved onto their territory.
"If just a single toecap of a Russian soldier steps into NATO territory then it would be war with NATO," he told Sky News.
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In another interview he said "it's not impossible" that Russia would launch an attack on Nato territory but he thinks it is "very unlikely".
The UK and other NATO countries have so far resisted sending troops into Ukraine amid fears it could cause a dramatic escalation in the conflict with nuclear-armed Russia.
It comes after a Russian opposition politician said Vladimir Putin will "destroy the world" if left unchecked and global leaders should brace for the Kremlin to deploy nuclear weapons.
Leonid Volkov, the former chief of staff for jailed Putin critic Alexei Navalny, issued the grim warning on Sunday - and said he wanted his fellow Russians to make it clear it was "not our war".
Mr Javid also accused Russia of committing a war crime by attacking a maternity hospital in Mariupol, with the WHO declaring that 31 atrocities have been committed at health facilities.
It emerged on Monday that a mother and baby who were pulled from the rubble have now died.
The image of the stricken mum being stretchered away from the destruction sent shockwaves around the world.
Mr Javid said: "In fact, the most recent information I have from the World Health Organisation is they now have documented evidence of at least 31 such attacks of health facilities hospitals, including shelling.
"A couple of days ago of a cancer hospital, as well. So these are war crimes."
Justice Secretary Dominic Raab will travel to The Hague today to offer Britain's help to gather the evidence of potential war crimes committed in Ukraine.
Boris Johnson will also host Nordic and Baltic leaders this week to discuss security as the Russian onslaught continues.
In a call with Ukrainian president Volodymyr Zelensky on Sunday, the Prime Minister said the UK would "continue to pursue more options for bolstering Ukraine's self-defence".
No10 said Mr Johnson told Zelensky that Russian president Vladimir Putin's "barbaric actions" were "testing not just Ukraine but all of humanity".
It comes as a new scheme to allow Brits to take Ukrainian refugees into their homes was due to be launched today.
Levelling Up Secretary Michael Gove is expected to set out more details in the Commons, before a website goes live allowing people to register their interest.
