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Boris Johnson says he will unveil a "first barrage" of economic sanctions against Russia later, after it sent troops into eastern Ukraine.
The PM said the UK would "hit Russia very hard" after it sent soldiers into two rebel-held regions it recognised as independent states on Monday.
He added there was "a lot more we are going to do" if a full-scale invasion of the country takes place.
He is expected to set out more detail in the UK Parliament later.
Mr Johnson said Russia had violated Ukraine's sovereignty and "torn up" international law, after it "sent troops in" to rebel-held Luhansk and Donetsk.
Russia has said the troops would be "peacekeeping" in the breakaway regions - a claim dismissed by the United States as "nonsense".
Speaking after an emergency meeting of cabinet ministers, Mr Johnson said "all the evidence" suggested Russian President Vladimir Putin was "bent on a full-scale invasion of Ukraine".
He said the UK sanctions announced on Tuesday would only be an initial step, as "we expect more Russian irrational behaviour to come".
He did not set out specific details, but said sanctions would target bodies not just in Luhansk and Donetsk, but in Russia itself.
He added they would hit Russian economic interests supporting "Russia's war machine" and its wider activities in the region.