The Nordic nation shares a 1,340-kilometre (830-mile) border with Russia and has remained militarily non-aligned since the end of World War II to avoid provoking its eastern neighbour.

But Russia's invasion of Ukraine on February 24 sent support for joining the US-led military alliance skyrocketing in Finland.

"The whole of Finnish society must be vigilant towards Russian attempts to influence Finnish decision-making regarding the NATO question," Antti Pelttari, head of the Finnish security services Supo, said.

Releasing its updated terrorism threat report, Supo on Tuesday highlighted the danger of "widespread Russian interference and illegal surveillance," but kept the national terror threat at level two, or "elevated", on a scale of four.

This weekend Finnish President Sauli Niinisto said attempting to join NATO could evoke Russian responses "on the brash side", including airspace or territorial violations.

Supo warned Finland's companies to be "constantly alert" to the threat of cyber-attacks disrupting power supply or other critical infrastructure.

Finland has previously been subject to so-called hybrid tactics from Moscow, such as repeated airspace incursions, or the release in 2016 of 1,700 migrants across the Finnish border.

Earlier this month the transport authority Traficom said it had received "numerous" reports from aircraft of GPS interference in eastern Finland, but was unable to identify the source of the interference.

President Niinisto has repeatedly called for Finland to decide "without hesitation but carefully" on NATO membership, insisting that the decision is one for the country's parliament, the Eduskunta.

Lawmakers are due to receive a government-commissioned report early next month setting out the risks and benefits of joining, as a precursor to a likely debate on whether to lodge a membership application.