Reports emerged of many people dead and injured, with children believed to be trapped under the rubble after a hospital in the besieged port city of Mariupol was severely damaged in the attack.
Ukrainian president Volodymyr Zelensky condemned the strike as an “atrocity” and reiterated his call to Western nations to impose a no-fly zone.
In response, Mr Johnson tweeted: “There are few things more depraved than targeting the vulnerable and defenceless.”
The strike came after Ukraine and Russia had agreed to hold a 12-hour ceasefire in six cities in Ukraine, meaning Mariupol should have been safe.
Mr Zelensky posted footage online showing the damage from what he said was a “direct strike” on the hospital, with windows blown out and debris strewn through the corridors.
“People, children are under the wreckage. Atrocity!” he tweeted.
“How much longer will the world be an accomplice ignoring terror? Close the sky right now! Stop the killings! You have power, but you seem to be losing humanity.”
Defence secretary Ben Wallace had earlier told MPs the UK has delivered 3,615 anti-tank weapons and would shortly be supplying a “small consignment” of Javelin anti-tank missiles.
Making a statement in the Commons, Mr Wallace claimed this was Ukraine's “darkest hour” and pledged to help.
He said: “In response to a Ukrainian request, the Government has taken the decision to explore the donation of Starstreak high-velocity manned portable anti-air missiles.
“We believe that this system will remain within the definition of defensive weapons, but will allow the Ukrainian force to better defend their skies.
“We shall also be increasing the supplies of rations, medical equipment and other non-lethal military aid.”
MPs heard the UK had already provided 3,615 anti-tank missiles, with more support to come including anti-tank Javelin missiles.
The anti-tank weapons, known as NLAWs, are made by the Swedish company Saab. With a range of up to 600m, the weapon is a portable, shoulder-mounted missile system weighing just 12.5kg.
The British Army says the missile can destroy “any main battle tank in just one shot by striking it from above”.
Mr Wallace stressed the Government was “bound by the decision to supply defensive systems” and not escalate the war.
The senior minister also warned the risk of Russian aggression against other countries is unclear, despite insisting Mr Putin was “contained in a cage of his own making”.
He said: “As long as we stand united, both as a House and as the international community, then the Kremlin’s threats cannot hurt us.
“President Putin and the Kremlin continue to threaten countries who offer help to Ukraine. Their military campaign will, I’m afraid, get more brutal and more indiscriminate.
“But it is my firm belief that our strength to stand up to such bullying comes from our alliances.”
Shadow defence secretary John Healey told the House of Commons the Government had Labour’s “full support” for reinforcing Nato nations on the eastern border with Russia.
Germany announced on Wednesday it won't be sending warplanes to Ukraine, after the US rejected an offer by Poland to transfer its Russian-made MiG-29 jets to Ukraine via a US base in Germany.
Chancellor Olaf Scholz insisted Germany has "provided all kinds of defence materials" including weapons, but "definitely warplanes are not part of that".
Earlier Downing Street said it was "not tenable" for Nato pilots and jets to be shooting down Russian planes.
The debate over military support came as the row over refugees coming to Britain saw a stunning intervention from Ukraine's ambassador to the UK Vadym Prystaiko.
Speaking at a committee, he told MPs his own wife had struggled to get a visa, and urged them drop visa rules for Ukrainians for a limited period, as Russia continues to invade.
The UN has said over two million people have fled Ukraine. As of Wednesday morning ,the UK had granted visas to 700 people.
This compares to 1.2 million refugees taken in by Poland, or 50,000 to Germany.
Enver Solomon, chief executive of the Refugee Council, urged ministers to urgently waive visa applications for people fleeing war-torn Ukraine in what he described as the “worst refugee crisis in Europe since the Second World War”.
He said: “I think it unfortunately sends the message to Ukrainians that ‘we’re not welcoming you’. We’re not creating a system that is quick, fair and efficient to enable those Ukrainians to come to the UK who want to come here.”
Home Office minister Baroness Williams of Trafford suggested a further 1,000 visas would be granted to Ukrainian refugees on Wednesday.
Speaking at joint press conference in Washington with the US Secretary of State Antony Blinken, Liz Truss warned there would be “terrible implications for European and global security” if Mr Putin was not stopped in Ukraine.
The UK foreign secretary said: “We know from history that aggressors only understand one thing, and that is strength.
“We know that if we don’t do enough now, other aggressors around the world will be emboldened.
“And we know that if Putin is not stopped in Ukraine, there will be terrible implications for European and global security. We would be sending a message that sovereign nations can simply be trampled on.
“So we must go further and faster in our response.”
Ms Truss said the Russian attack on a hospital was “absolutely abhorrent”, but continued to reject Ukraine’s request for a no-fly zone to be imposed in its skies.
She explained: "The best way we can protect the skies is through anti-air weaponry, which the UK is now going to be supplying to Ukraine.
“What we’re looking at is making sure that the Ukrainians are able to defend their open country with the best possible selection of anti-tank weapons and anti-air defence systems.”
Mr Blinken said it was possible to draw parallels between Britain’s experience in the Second World War and what was happening today in Ukraine.
He said: “I believe one reason we are seeing such an outpouring of support from the British people is because they’ve been through something similar.
“The harrowing Blitz during the Second World War inflicted colossal suffering on the country’s people, killing more than 60,000 British civilians, wounding 86,000 more.
“It is impossible to see the images of people seeking refuge in Kyiv’s metro in 2022 and not think of those who sheltered in the London Underground in 1942.
“The grit, the compassion and determination that Britain exhibited eight decades ago that inspired the world is exactly what we see in the people of Ukraine today, and it is why we need to stand with them.”
He added US involvement in installing a no-fly zone over Ukraine could “prolong” the conflict, expand it and make it “even deadlier”.