Russia suspended electricity supplies to Finland overnight after its energy firm RAO Nordic claimed payment arrears, an official for Finland's grid operator told AFP on Saturday.

"It is at zero at the moment, and that started from midnight as planned," Timo Kaukonen, manager for operational planning at Fingrid, said.

RAO Nordic had Friday said it would suspend supplies, citing problems with payments, as Helsinki prepares to announce its application for NATO membership in the wake of Moscow's invasion of Ukraine.

But the shortfall was being made up by imports from Sweden. Finland imports about 10 percent of its power from Russia.

RAO Nordic says it has not been paid for electricity since May 6 but has not spelt out if this was linked to European sanctions against Russia. 

Moscow's February 24 invasion has swung political and public opinion in Finland and Sweden in favour of NATO membership as a deterrent against Russian aggression. 

Both countries have long cooperated with the Western military alliance and are expected to be able to join it quickly. 

NATO chief Jens Stoltenberg has repeatedly said they would be welcomed "with open arms".

Finland is expected to formally announce its plan to join Nato on Sunday. 

For many Finns living on the eastern border, the prospect of their country applying to join NATO has been greeted with relief. 

Sharing a 1,300-kilometre (800-mile) border with Russia, Finland has in the past stayed out of military alliances.

But after its powerful eastern neighbour invaded Ukraine in February, political and public opinion swung dramatically in favour of membership, with the Finnish president and prime minister on Thursday calling for the country to join NATO "without delay".

For some Finns, Russia’s assault on Ukraine has brought up painful memories of the 1939 Winter War, when Red Army troops invaded the Nordic country. 

As in Ukraine, the small Finnish army put up a fierce resistance and inflicted heavy losses on the Soviets.

Nevertheless, Finland had to cede vast areas of land to the Soviet Union.