Finland, which dropped decades of military non-alignment to join NATO in 2023, has a 1,340-kilometre (830-mile) border with Russia.

Satellite images published in the New York Times appear to show an expansion of Russian military infrastructure near the border.

"Russia is building more infrastructure to be able to bring in more troops after the war (in Ukraine) is over," the Finnish Defence Forces told AFP.

Together with its allies, Finland is "closely monitoring and assessing Russia's activities and intentions", Defence Minister Antti Hakkanen told AFP in an email.

He said Russia's moves "to strengthen its armed forces have not come as a surprise to Finland".

Moscow has repeatedly warned Finland of repercussions since joining NATO after Russia's full-scale invasion of Ukraine in 2022.

"We have excellent capabilities to observe Russian operations. As a member of the alliance, Finland holds a strong security position," Hakkanen said.

He said Finland had a "robust national defence" based on conscription and a large reserve force.

"Our entire society has invested broadly in preparedness for all types of disruptions and crises," he added.

Defence Forces Deputy Chief of Staff for Strategy, Major General Sami Nurmi, told Finnish broadcaster Yle that Russia's buildup near the border was expected.

"That is consistent with what has been assessed. It is not a question of a mass construction effort," he told Yle.

He said the activity did not pose an immediate threat to Finland.

The Nordic country has beefed up its military investments and preparedness since joining NATO.

In April, it announced it would boost defence spending to at least three percent of GDP by 2029 and launch a reform of its defence forces to tackle the security threat.

Finland shut its border with Russia in December 2023 after the arrival of around 1,000 migrants without visas. Helsinki said the surge was orchestrated by Russia -- a claim Moscow denied.