The Prime Minister visited a Ukrainian cathedral in London on Sunday in a show of support for Ukraine after the Russian invasion.
Boris Johnson told the faithful at the Cathedral of the Holy Family that the invasion represented "the darkest times in modern memory" and that he had never seen "so clear a distinction between good and evil".
He promised that the UK would do "everything it can to help economically, politically, diplomatically, militarily".
"When innocent people are subject to a barbaric and unprovoked attack, we in the UK cannot shut our eyes and pass by on the other side," he said.
Speaking of the UK's "emotional", "political" and "strategic" bonds with Ukraine, the Prime Minister said it was "right" to help the country in its "hour of need".
But the Prime Minister also made a distinction between the people of Russia and the actions of the country's leadership in choosing a "path of violence".
"I'm going to stress that there is no hostility in my heart towards the Russian people, none whatever - quite the reverse," he said.
"My heart aches for the Russian parents who are already losing their children in this vicious and appalling war - just as it aches for the civilians and the people of Ukraine."
The Russian invasion is now in its fifth day after encountering fierce resistance from the Ukrainian military.
Peace talks were to take place on Monday on the border between Ukraine and Belarus.
Ukrainian President Volodymyr Zelensky has said the next 24 hours will be a "crucial period" for his country.