Vladimir Putin is “doubling down” on the use of indiscriminate attacks on Ukraine’s towns and cities, Boris Johnson said.
The Prime Minister said the “unrestrained” acts of aggression were resulting in “huge waves” of refugees being forced out of their homes.
The UK has faced criticism for refusing to open its borders to Ukrainians but Mr Johnson insisted the country would welcome those fleeing the fighting, with thousands of visa applications being processed.
In London, Mr Johnson was beginning a week of intense diplomatic efforts with foreign leaders to build a united front against Mr Putin, hosting Canadian Prime Minister Justin Trudeau and Dutch Prime Minister Mark Rutte.
He is due to hold talks with US President Joe Biden later on Monday.
Speaking at RAF Northolt in west London, Mr Johnson said: “Clearly, what’s happening now is that Putin is doubling down on his aggression and he is deciding to attack in a pretty indiscriminate way.
The Home Office revealed that only “around 50” visas had been granted under the Ukraine Family Scheme as of 10am on Sunday but Mr Johnson said that “thousands” of applications were being processed.
As well as the visas on offer to those with family in the UK, a humanitarian sponsorship scheme will allow people and organisations to bring Ukrainians to the UK.
The Prime Minister said: “As the situation in Ukraine deteriorates, people are going to want to see this country open our arms to people fleeing persecution, fleeing a war zone.
Rockets continued to rain down on Ukrainian towns and cities and Mr Johnson indicated the West’s sanctions on Mr Putin and his regime would be stepped up.
“Now that he is going for this really unrestrained attack on cities, now that he’s attacking civilians in the way that he is, I think we’ve got to recognise that we’ve got to do more on sanctions,” the Prime Minister said.
There was also “more to be done” on targeting individuals linked to Mr Putin’s regime, Mr Johnson said, although he stressed the need to avoid a “witch hunt” against all prominent Russians in the UK.
The Government was pushing a new Economic Crime Bill through the Commons on Monday in a fast-track procedure to bolster its ability to target oligarchs.
The Russian authorities announced the new humanitarian corridors in Ukraine on Monday, saying civilians would be allowed to leave the capital of Kyiv, the southern port city of Mariupol, and the cities of Kharkiv and Sumy.
Previous attempts to create humanitarian corridors have ended with civilians being shelled as they tried to flee to safety.
Mr Cleverly told the BBC: “It appears cynical beyond belief. There is a view that Vladimir Putin believed there was a widespread desire of Ukrainians to be closer to Russia, to be more Russian. I think that has been proven to be a complete nonsense by the circumstances we are seeing.
“Providing evacuation routes into the arms of the country that is currently destroying yours is a nonsense.”
He added that “ultimately the most humanitarian thing the Russians could do is end this completely illegal, completely unjustified invasion of Ukraine”.