'Barbaric' Vladimir Putin is killing rising numbers of children, Boris Johnson declared today after touching down for crisis talks.
The Prime Minister arrived in Poland where he warned the Russian war in Ukraine "is worse than our predictions", "we’re seeing an unfolding disaster in our European continent."
Standing beside Poland’s Prime Minister Mateusz Morawiecki, he said: "Vladimir Putin is prepared to use barbaric and indiscriminate tactics against innocent civilians.
"To bomb tower blocks, to send missiles into tower blocks, to kill children, as we are seeing in increasing numbers".
It comes as Kyiv's ambassador to Britain warned MPs of the drastic tactics Putin could deploy, including starving the Ukrainian people.
And Lithuanian Foreign Minister Gabrielius Landsbergis warned it was clear Putin has "no boundaries" to what instruments he will use against Ukraine.
“It is possible that this war will become a massacre," he told the UK’s Foreign Affairs Committee.
Mr Johnson told Polish PM Morawiecki “this is a key moment for the Ukrainian resistance and we must be ready to increase and intensify sanctions”. And he "praised Poland for being on the front line of humanitarian efforts."
In a speech in Warshaw afterwards, Mr Johnson warned “Putin will continue to tighten the vice”.
He said Vladimir Putin was "tearing up every principle of civilised behaviour between states".
"He has hurled his war machine on the people of Ukraine, a fellow Slavic country, he has bombarded civilian targets, fired rockets at blocks of flats, he is responsible for hundreds of civilian casualties including growing numbers of children.
"And also, of course, for the deaths of many Russian and Ukrainian soldiers.
"We must accept the grim reality that Putin will continue to tighten the vice and, if you go by the size and firepower of Vladimir Putin's war machine, the odds have always been heavily against Ukrainian armed forces."
But he insisted Vladimir Putin had made a "colossal mistake" by invading Ukraine.
The PM was challenged by a Ukrainian journalist who made an emotional appeal for a no-fly zone to protect civilians in the country.
"That's not something that we can do or that we've envisaged. The consequences of that would be truly very, very difficult to control," he said.
Mr Johnson appeared to confirm more help for refugees as he faces mounting pressure to widen the UK's offer.
“We will make it easier for Ukrainians already living in the UK to bring their relatives to our country," he said. "And though the numbers are hard to calculate, they could be more than 200,000."
He added: “We’re 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’re 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.”
Sources suggested the PM was extending the family visa waiver scheme - previously only available to close relatives - and that the sponsorship route was new.
He added if Ukraine's government does collapse, “we must prepare to support them whatever happens in the weeks ahead”.
But he added: “Putin has stubbed his toe and tripped on a giant and immovable fact - which is no matter how many troops and tanks he sends, the Ukrainians desire to live in a free and independent country.”
He is facing anger after, so far, only waiving visas for the closest Ukrainian family members of people already living in Britain.
Around 100,000 are eligible for the UK's current offer - but the EU plans to offer fleeing Ukrainians the right to stay and work for up to three years.
And the 100,000 total is thought to be the entire total of potentially eligible close family members in Ukraine - not an estimate of how many will actually come.
Home Secretary Priti Patel said last night that she will announce "further changes" in "the next few days".
Ms Patel told ITV: “We're absolutely working on this, because we just don't know what tomorrow will bring.
“Clearly alongside that though, I must emphasise this, we are working with the Ukrainian government because primarily you know people want to stay in the region."
But shadow home secretary Yvette Cooper told BBC Radio 4's Today programme: "At the moment, what the Home Office is doing is trying to just tweak the existing system...
"And the normal system just doesn't work when you are facing war in Europe."
In Poland, Boris Johnson was meeting Prime Minister Morawiecki - before meeting NATO's Secretary General and UK troops in Estonia.
The Prime Minister was also due to meet Estonian Prime Minister Kaja Kallas and Estonian President Alar Karis this afternoon.
It comes after Oksana Markarova, Ukraine's ambassador to the US, said Russia had used a thermobaric weapon, known as a vacuum bomb, in its invasion of her country.
A vacuum bomb sucks in oxygen from the surrounding air to generate a high-temperature explosion that is capable of vaporising human bodies.
Concerns had been growing last night among Western officials that the weapons could be deployed.
They raised “significant concern” last night about evidence the TOS-1A thermobaric rocket launcher was present among Russian forces.
A Western official said: “Given the indiscriminate nature of that system, and it is certainly if it’s used in any form of built-up area, there is no way in which you could eradicate the risk of significant civilian casualties through the use of those sorts of systems.”
A Russian military convoy around 40 miles long is north of Kyiv and there are fears that the city will face a barrage of rockets and shells.
Deputy Prime Minister Dominic Raab, a former Foreign Office lawyer, said it must be clear "both to Putin but also to commanders in Moscow and on the ground in Ukraine that they will be held accountable for any violations of the laws of war".
Britain's UN Ambassador Dame Barbara Woodward has warned Ukraine is on the brink of a "humanitarian catastrophe".
And Foreign Secretary Liz Truss was due to say Putin has “blood on his hands” in an address to the UN Human Rights Council in Geneva just before lunchtime.
She was due to say: “Tanks are tearing through towns while missiles barrage homes and hospitals. Putin is murdering Ukrainians indiscriminately.
"There is blood on his hands, not just of innocent Ukrainians but the men he sent to die.
“Putin is violating international law, including the UN Charter. He is violating human rights on an industrial scale and the world will not stand for it.”
Dominic Raab warned Vladimir Putin could respond with "even more barbaric tactics" and “we must be prepared that this could be a long haul”.
The Deputy Prime Minister did not rule out supplying Ukraine with fighter jets to defend itself against Russia, telling LBC: “We've been clear that we've provided training, we've provided anti-tank weapons and all I would say is nothing is off the table."
But he continued to rule out the UK enforcing a no-fly zone to aid Ukraine's resistance to the Russian invasion.
He told BBC Radio 4's Today programme: "Putting UK forces in the position where they would be directly required to shoot down Russian planes, I agree with the analysis... both in terms with feeding (Vladimir) Putin's narrative, but also from the point of the view of the distinction between what we would do for a Nato ally and the Ukraine, which is a close partner, we want to support, but we will not get involved directly in military operations against Russian forces.
"We have considered a no-fly zone but, for the reasons I've given, the international community as a whole has decided against it."