"This is an innocent people who are facing a totally unprovoked act of aggression against them, and what's actually happening is that they are fighting back, perhaps with more effect, with more resistance, than the Kremlin had bargained for," he said.

When asked about Putin's decision to put nuclear forces on alert, he said: "That's a distraction from the reality of what's going on in Ukraine."

Johnson also shed doubt on Putin's sincerity to meet Ukraine for talks, after Ukrainian President Volodymyr Zelensky said he was ready to "try" talks.

"There's nothing I've seen so far in his behaviour that leads me to think that he could possibly be sincere," the UK prime minister said, referring to Putin.

He called Russia's invasion of Ukraine a "continuing disaster", but said Moscow would not succeed.

"I've got absolutely no doubt in my mind that, in the end... I think Putin will fail.

"He will not succeed in crushing Ukraine and we in the UK will do everything in our power to ensure that outcome," he added.

Britain has joined Western nations in slapping sanctions on Russia after its invasion of Ukraine on Thursday.

On Sunday, the government announced pound40 million ($54 million, 48 million euros) in fresh aid for Ukraine to "help aid agencies respond to the deteriorating humanitarian situation", Johnson's office said in a statement.

The aid would create "a lifeline for Ukrainians with access to basic necessities and medical supplies such as medicines, syringes, dressings and wound care packs", it said.

"UK government humanitarian experts have also deployed to the region to support those fleeing the violence in Ukraine," it added.