Finnish president Sauli Niinisto is in hospital for monitoring due to "prolonged Covid-19 symptoms," his office said Tuesday. "President Niinisto is feeling reasonably well and is continuing to work remotely," the office said.

The 73-year-old head of state was diagnosed with coronavirus on April 19, leading to the postponement of a visit to Nordic neighbour Norway.

The president's illness comes as Finland's politicians mull a potential application to join NATO, which would be a historic reversal in policy for the country.

Finns have traditionally been against joining the Western military alliance, but opinion polls have recorded soaring support and even majorities in favour following Russia's invasion of Ukraine.

Niinisto is scheduled to visit neighbouring Sweden, where the issue is also being debated, on May 18, and the two countries are widely expected to act in unison.

Since the end of the Cold War, the two Nordic nations have remained non-aligned, but they have deepened ties and are close partners with NATO, stopping short of membership.

Finland's Prime Minister Sanna Marin, on a visit to Sweden in mid-April, said a decision on a membership application was expected "within weeks."