Helsinki was quick to condemn Moscow's recognition of the pro-Russian separatist republics of Donetsk and Lugansk in eastern Ukraine on Monday evening.

Asked at a press conference on Tuesday whether fears of a possible Russian invasion of Ukraine had brought Helsinki closer to joining the US-led security alliance, Niinisto said he did not know.

But "we have a lively debate on NATO membership, and it will certainly be further activated if Russia gives a reason for it," he said.

"Finland strongly supports Ukraine and the Ukrainian people," added the Finnish leader, who has had regular contact with Russia's President Vladimir Putin as tensions have mounted in recent months.

Finland and neighbouring Sweden are not members of NATO, but both countries are partners of the Western military alliance.

Faced with Moscow's demands that NATO not expand eastwards, Helsinki and Stockholm have said they refuse any Russian interference in their security policy.

Finland and Sweden have received assurances from NATO that the door remains open to them, though the Social Democrats in power in both countries have no plans to join.

Finland officially considers an application for membership as an "option", unlike Sweden.

But, Niinisto told reporters, even if the country did decide on an application, procedures could be lengthy and NATO member states would still need to agree.

"There is no flash process," he said.

A part of Russia between 1809 and 1917, Finland shares a border of more than 1,300 kilometres (800 miles) with its eastern neighbour.

The two countries clashed militarily during the Second World War.

During the Cold War, Finnish leaders agreed to stay out of Western defence cooperation to avoid a Soviet invasion.