The Prime Minister insisted no Nato allies were contemplating heeding Ukraine’s pleas to enforce a no-fly zone, warning it would trigger a wider war with Russia.
The rejection was issued as satellite images showed the extent of the Russian forces massing around Kyiv, with a column of armoured vehicles, tanks, artillery and support vehicles stretching 40 miles advancing on Ukraine’s capital.
Russia separately attacked a 380m-tall TV tower in Kyiv, with Ukrainian emergency services claiming five people were killed in the strike.
Foreign secretary Liz Truss also announced a first tranche of sanctions against Belarusian individuals and organisations in response to the role the country was playing in Russia’s invasion of Ukraine.
Four senior defence officials and two military enterprises have been sanctioned with immediate effect under the UK’s Russia sanctions regime, the Foreign, Commonwealth and Development Office said.
Meeting with leaders in Poland and Estonia, Mr Johnson warned those involved in the “Russian onslaught” that evidence was being gathered for a possible trial at the International Criminal Court.
The Prime Minister again rejected Ukrainian president Volodymr Zelensky’s calls for British forces to actively join the effort, after being challenged by a Ukrainian journalist at a press conference over the UK’s decision not to commit to the no-fly zone.
Speaking at the Tapa military base in Estonia, he said: “I think for any Nato member to get involved actively in conflict with Russia is a huge step, which is not being contemplated by any member.
“This is a time when miscalculation and misunderstanding is all too possible and it’s therefore crucial that we get that message over.
“When it comes to a no-fly zone in the skies above Ukraine, we have to accept the reality that that involves shooting down Russian planes.
“That’s a very, very big step. It’s simply not on the agenda of any Nato country.
“We will not fight Russian forces in Ukraine. Our reinforcements, like these reinforcements here in Tapa, are firmly within the borders of Nato members.”
In an earlier visit to Warsaw, Poland, a Ukrainian journalist who fled over the border made an impassioned plea for Mr Johnson to assist with the no-fly zone.
Daria Kaleniuk, the executive director of the Anti-Corruption Action Centre civil society organisation, said: “Nato is not willing to defend because Nato is afraid of World War Three, but it’s already started and it’s Ukrainian children who are there taking the hit.”
Mr Johnson apologised as he ruled out the move, having blamed Mr Putin’s regime for “barbaric and indiscriminate” violence against Ukrainian citizens.
He said: “On Putin’s tactics, there’s no doubt that he’s already using barbaric tactics, bombing civilian areas. I’ve seen the reports about cluster bombs and thermobaric weapons. They will, of course, have to be verified.
“But I think that everybody involved in the Russian onslaught should understand that all this will be collated in evidence to be used at a future time in what could be proceedings before the International Criminal Court.
“I hope people understand that if you’re going to use illegal weapons against innocent civilians, you’re going to be brought to the bar of history, or rather to the International Criminal Court before then. I hope that he gets that.”
The Ukrainian leadership has already accused Russia of war crimes over the bombardment of civilian areas in Ukraine’s second largest city Kharkiv.
Ireland’s minister for foreign affairs Simon Coveney has said there is “indisputable evidence” of Russian war crimes in Ukraine.
He said: “We’ve seen I think indisputable evidence now that war crimes are taking place in Ukraine – the brutalising of a number of Ukrainian cities, the deliberate targeting of civilians.
“First of all, it is now under investigation from the International Criminal Court, who have now opened a file on potential war crimes in Ukraine, and I think that says a lot.
“But I think we can see for ourselves in terms of some of the social media pictures that are coming out, which I think are being stood over by the media organisations, as to the extent of civilian targeting in Ukrainian cities, which is essentially terrorising civilian populations.
“And that is something that can’t go unchecked and we need to call it out.”
The UN says at least 136 civilians have been killed so far, including 13 children, and hundreds more injured.
Mr Zelensky told the European Parliament 16 children had been killed as he appealed to EU leaders to allow his country’s accession to the bloc.
Satellite images overnight had shown the extent of the Russian forces massing around Kyiv.
The column of armoured vehicles was 17 miles from the capital on Monday, but is believed to have stalled through Tuesday due to supply issues.
Nato secretary general Jens Stoltenberg claimed the column of “heavy Russian armour” moving towards Kyiv would bring “more death, more suffering and more civilian casualties”.
He said: “That is the reason why we need to continue to provide support to Ukraine, why we continue to call on Russia to stop this bloody war and why we need to impose costs by the heavy sanctions on Russia.”
Mr Stoltenberg said the situation was “horrifying” and “a blatant violation of international law”.
It came as the UK Government committed to doing more to allow Ukrainians to come to the UK, with around 200,000 eligible under an expanded route for people to bring family members in.
Home secretary Priti Patel refused to waive visas for those coming in, but expanded the entry scheme to more relatives of those settled in the UK, and also announced a new sponsorship programme.
Mr Johnson said: “What we are going to do is we are extending the family scheme so that actually very considerable numbers would be eligible. You could be talking about a couple of hundred thousand, maybe more.
“Additionally, we are going to have a humanitarian scheme and then a scheme by which UK companies and citizens can sponsor individual Ukrainians to come to the UK.”
Meanwhile, there was a mass walkout of diplomats from the UN Human Rights Council when a speech from Russian foreign minister Sergei Lavrov began.
Ms Truss said Mr Lavrov’s statement was “full of disinformation” and did not deserve the attention of other members.
“Russia is isolated and should be ashamed to sit in the UN chamber,” she said.
During her own speech to the Geneva meeting, Ms Truss said: “Putin is responsible for civilian casualties and over 500,000 people fleeing with the numbers still rising fast. The blood is on Putin’s hands – not just of innocent Ukrainians, but the men he has sent to die.
"We're using our collective heft, making up over half the world's economy to cut funding from Putin's war machine and we're delivering severe economic costs through these sanctions as ordinary Russians are finding form queues at their local banks and rising interest rates.'
“These consequences will only increase in breadth and severity as the conflict goes on, we're working to squeeze the Putin regime harder and harder by steadily tightening the vice.
“We're going after the highest echelons of the Russian elite, targeting President Putin personally and all of those complicit in his aggression. Nothing and no one is off the table.”
Some in the UN Security Council are believed to be pushing for Russia to be suspended from it, but a Foreign Office minister has claimed that could only happen if Russia voted in favour of it.