A Russian army helicopter has violated Finland's airspace, the Finnish defence ministry said, as the country mulls an increasingly likely NATO membership bid.

"The aircraft type is a Mi-17 helicopter and the depth of the suspected violation is about four to five kilometres", a ministry spokesman said.

The incident occurred this morning around 7.40am Irish time.

This is the second Russian airspace violation this year, following a previous one in April, both of them coming in the wake of the war in Ukraine.

A civilian transport plane belonging to the Russian army briefly entered Finnish airspace on 8 April.

Experts have warned that Finland and Sweden would likely be subjected to Russian acts of interference as they consider whether to join NATO as a deterrent against aggression from their eastern neighbour.

Four Russian fighter jets violated Swedish airspace in early March near Sweden's strategically-located island of Gotland in the Baltic Sea.

And on Friday, a Russian spy plane also crossed the Swedish border near a naval base in the south of the country.

Hostile acts such as cyberattacks are also considered likely, according to analysts, who have however said that the risk of a military attack was low.

A large majority of Finnish MPs and the public are in favour of joining the Atlantic Alliance, and Finnish President Sauli Niinsto is to announce his personal stance on the issue on 12 May.

Finnish Prime Minister Sanna Marin said today her government's decision would be made "quite soon", and could come within days according to Finnish media.

Finland was ruled by Russia for 108 years before declaring independence in 1917.

It fought off a Soviet invasion during World War Two before a peace deal saw it cede several border areas to Moscow.

The Nordic nation remained neutral during the Cold War in exchange for Soviet guarantees not to invade.