Finland accused Russia on Thursday of violating its airspace for the third time in less than a week, saying it would demand an explanation for what the defence minister termed an unacceptable incursion.
Finland scrambled jets to identify the Russian plane it suspected of crossing over, which it called a state aircraft -- a term that refers to those used in military, customs and police services.
"We will express the fact that from our perspective, this is totally unacceptable," minister Carl Haglund said according to the online edition of newspaper Helsingin Sanomat.
The Russian defence ministry was not immediately available for comment.
Haglund's remarks came as Ukraine's president accused Russia on Thursday of bringing troops across the countries' border in support of pro-Moscow separatist rebels.
Haglund said the frequency of the incidents suggested the breaches of Finnish airspace were intentional.
Helsinki accused Moscow of similar incursions on Saturday and Monday, with the Finnish border guard saying the latter episode was a clear violation. It describes incursions as suspected until it has analysed all relevant data.
"It is very difficult to see it as a matter of coincidence. Unfortunately, due to the lack of valid explanations, it seems there is some intentionality involved," Haglund told the newspaper.
He also said Finland would step up airborne monitoring of flights within its airspace.
In all, Finland has now accused Russia of violating its airspace five times since the Ukraine crisis started in February, although the border agency concluded that two instances in May were likely to have been accidental.
Such violations usually happen a few times each year, but three within a week is highly unusual.
Finland shares a 1,300-km (800-mile) border with Russia and maintains generally cordial relations with its former ruler, but Finland's defence forces are reported to have stepped up surveillance due to the Ukraine crisis.
On Wednesday, Finland increased its cooperation with the NATO Atlantic alliance, entering into a framework arrangement to outline cooperation in emergencies.