British Prime Minister Boris Johnson on Thursday said Russian President Vladimir Putin was a "dictator" who now faced "massive" Western sanctions for invading Ukraine.

"We cannot and will not just look away," Johnson said in a televised address to the nation, after phoning Ukraine's President Volodymyr Zelensky just after 4:00 am (0400 GMT) as Russian forces moved in.

Ukraine can be assured of continued UK support given that "our worst fears have now come true and all our warnings have proved tragically accurate", the prime minister said.

Ahead of an emergency virtual meeting of G7 leaders, Johnson said the West "will agree a massive package of economic sanctions designed in time to hobble the Russian economy".

"And to that end we must also collectively cease the dependence on Russian oil and gas that for too long has given Putin his grip on Western politics," he added.

"Diplomatically, politically, economically -- and eventually, militarily -- this hideous and barbaric venture of Vladimir Putin must end in failure."

Johnson was unusually direct in highlighting Putin's personal role in bringing war back to Europe, calling it "an attack on democracy and freedom in east Europe and around the world".

The "flame of freedom" would return in time to Ukraine, he said.

"Because for all his bombs and tanks and missiles, 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."

Johnson summoned his security chiefs for an early-morning meeting in response to the Russian invasion, and was also to address parliament at 5:00 pm (1700 GMT).

Unprecedented' sanctions 

UK Foreign Secretary Liz Truss, who spoke to her US counterpart Antony Blinken Wednesday evening before Putin announced the start of military operations, joined Johnson in condemning the attack.

The foreign ministry has deployed teams to five countries in eastern Europe to support Britons leaving Ukraine, she noted. 

Meanwhile Transport Secretary Grant Shapps said he had instructed the UK Civil Aviation Authority to ensure airlines avoid Ukraine airspace "to keep passengers and crew safe".

The UK slapped sanctions Tuesday on five Russian banks and three billionaires, in what Johnson called "the first barrage" of measures in response to the Kremlin's actions in Ukraine.

Leading members of Johnson's ruling Conservatives, as well as the main opposition Labour party, have urged him to hit the Kremlin as hard as possible with the new sanctions.

Foreign office minister James Cleverly vowed London would respond with "unprecedented" steps "to punish this aggression".

"Those sanctions will be laid today and over forthcoming days to really prevent Russia from funding this invasion," he told the BBC.

"The sanctions package that will be put in response to this is already actually having an effect," Cleverly added, noting record falls Thursday on the Russian stock market and a slump in the ruble's value.