Prime Minister Petteri Orpo on Tuesday said Finland will close its final remaining border crossing with Russia, accusing the Kremlin of using migrants as part of "hybrid warfare" aimed at destabilizing the Nordic country following its entry into NATO.

This month, Finnish authorities say about 900 crossed from Russia, an unusual increase, but as part of a rising trend since August.

"The government has decided to close all the crossing points on the entire eastern border," Prime Minister Petteri Orpo told reporters, adding it was an "exceptional" situation that the government blames on Moscow.

'Russia caused this situation,' says PM

"Finland has a profound reason to suspect that the entry (of migrants) is organized by a foreign state. This deals with Russia's influencing operations and we won't accept it," Orpo said.

"We don't accept any attempt to undermine our national security. Russia has caused this situation and it can also stop it."

Finland had already closed seven of its eight checkpoints on the 1,340-kilometer (830-mile) long stretch it shares with Russia.

Helsinki has accused Moscow of encouraging migrants from the Middle East and Africa toward the border through thick forests in the south to the rugged Arctic landscape in the north.

Russia denies Finnish allegations amid migrant spike   

Since August, around 1,000 migrants have entered Finland, without visas or valid documentation. People from countries including Afghanistan, Eritrea, Ethiopia, Iraq, Pakistan, Somalia, Syria and Yemen, have come to the border, most of them seeking asylum.

Finnish Foreign Minister Elina Valtonen said that Russia "is instrumentalizing migrants" as part of its "hybrid warfare."

Before joining NATO this year, a decision prompted by Moscow's invasion of Ukraine, Finnish officials warned that Russia could seek ways to destabilize the country in retaliation, including cyber attacks and ushering migrants to its borders.

"We have proof showing that, unlike before, not only Russian border authorities are letting people without proper documentation to the Finnish border, but they are also actively helping them to the border zone," Valtonen said.

The Kremlin denies prompting the migrants, and says it regrets Helsinki's move to close the borders. Last week, Russian Foreign Ministry spokeswoman Maria Zakharova said that Finland should have tried to "work out a mutually acceptable solution or receive explanation."

jsi/wd (AP, AFP, dpa)