Prime Minister Boris Johnson said Britain and its allies would unleash a massive package of economic sanctions to hobble the Russian economy after the Kremlin launched an all-out invasion of neighboring Ukraine on Thursday.

Western nations are expected to announce coordinated sanctions after earlier this week imposing a limited initial package that was criticized by some as a weak response to Russia's recognition of two breakaway regions in Ukraine.

"Today, in concert with our allies, we will agree a massive package of economic sanctions designed in time to hobble the Russian economy," Johnson said in a televised address to the nation.

He said the West must end its reliance on Russian oil and gas which had given Russian President Vladimir Putin a grip over Western politics.

"Our mission is clear: diplomatically, politically, economically, and eventually military, this hideous and barbaric venture of Vladimir Putin must end in failure."

In earlier comments on Twitter, the British leader called the invasion a "catastrophe" for Europe, and said he would talk to other G7 groups of rich nations.

"I will also speak to fellow G7 leaders and I am calling for an urgent meeting of all NATO leaders as soon as possible," he said.

Foreign minister Liz Truss said she had summoned the Russian ambassador to explain Moscow's actions in Ukraine.

Britain, like the United States and European Union, had threatened to impose tougher sanctions on Russia if it invaded Ukraine, a move Russian President Vladimir Putin had earlier said Moscow would never do.

On Wednesday, Johnson told finance chiefs he wanted to impose the "toughest possible next tranche" of sanctions on Russia, an action he described as being able to "make a difference and change the outcome."

In his address on Thursday, he told Russians he did not believe the invasion was being carried out in their name, while he vowed to support Ukraine until the flame of freedom "burns bright again."

"I don't believe that the Russian dictator will ever subdue the national feeling of the Ukrainians and their passionate belief that their country should be free," he said.

"I say to the British people, and all who have heard the threats from Putin against those who stand with Ukraine: We will of course, do everything to keep our country safe."