PARIS: France will respond without weakness to Russia's act of war against Ukraine, said President Emmanuel Macron on Thursday, adding that Russia could expect tough sanctions that would hit its military, its economy and its energy sector.

"The events of last night mark a turning point in the history of Europe," said Macron in a televised address to the French nation, standing before French, European Union and Ukranian flags.

"We will respond to this act of war without weakness, with cold blood, determination and unity," he added.

French Foreign Minister Jean-Yves Le Drian had also earlier said that France would reinforce its support to Ukraine "in all its forms", without further specifying whether or not this could include military means.

NATO activating 'defence plans' for allies as Russia invades Ukraine

Russian forces invaded Ukraine on Thursday, staging an assault by land, sea and air in the biggest attack by one state against another in Europe since World War Two.

British Prime Minister Boris Johnson, speaking shortly before Macron in a televised address from Downing Street, had also said that Britain and its allies would unleash a massive package of economic sanctions to hobble the Russian economy.

"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.

Macron hopes for rewards but faces risks in Moscow trip to see Putin

Western leaders are due to discuss further steps during several high-level formats on Thursday, including an emergency NATO meeting.