Russian President Vladimir Putin has warned his Finnish counterpart that joining Nato and abandoning Finland's neutral status would be a "mistake".
He told Sauli Niinist√∂ there was no threat to Finland's security.
The exchange came during a phone call made by the Finnish president, ahead of a formal request which Finland is expected to announce very soon.
Sweden has also indicated its intention to join the Western alliance, following Russia's invasion of Ukraine.
Finland shares a 1,300-km (810-mile) border with Russia. Until now, it has stayed out of Nato to avoid antagonising its eastern neighbour.
Mr Putin did not make a specific threat of retaliation over Finland's move, but the Russian foreign ministry has indicated there will be retaliation.
Russia's decision to suspended electricity supplies to Finland is being seen as an early sign, however.