Finland said on Thursday that it was closing part of its border with Russia after a dramatic increase in migrant crossings that it blamed on Moscow, the latest sign of deteriorating relations between the neighbors since the Russian invasion of Ukraine.
The announcement followed days of warnings from the authorities in Finland over a spike in the number of crossings, which President Sauli Niinisto of Finland on Wednesday suggested was retaliation from Moscow for Finland’s decision to join NATO.
Prime Minister Petteri Orpo said at a news conference on Thursday that Finland was closing crossings on its eastern border starting at midnight on Friday. The government wanted to “react strongly,” he said, to what it viewed as “organized activity” by Russia.
“Finland as an E.U. and NATO member is steadily one of the countries that condemn Russia’s invasion of Ukraine,” he said. “Therefore we have been prepared for various action from Russia, nastiness, and this situation does not come to us as a surprise.”
He added: “We have acted decisively and promptly in order not to have the situation deteriorate.”
Speaking alongside Mr. Orpo, the interior minister, Mari Rantanen, said that the four crossings would be closed because issues on the border posed a threat to the country’s national security.
The Russian deputy foreign minister, Yevgeny Ivanov, called the accusations “very strange” and told the state news agency Tass that Moscow would “get to the bottom” of it.
Finland, a country of some 5.6 million, shares an 830-mile-long border with Russia — and also a combative history. The two countries have fought numerous wars through the centuries, and Finland has strong memories of the 1939 “Winter War” and World War II. It beat back the Soviets but lost territory and adopted form of neutrality in the face of the threat from the Soviet Union.
Russia’s invasion of Ukraine raised fears in Finland that it could be one of Moscow’s next targets. Its government cast aside decades of military nonalignment and quickly moved to join NATO, a step that Russia described as “clearly hostile.”
But even before the invasion, the issue of migrants was a sore point between Helsinki and Moscow. In late 2015 and early 2016, Finland experienced a surge of migrants seeking asylum crossing the Russian border, most of them from third countries. Then, too, Finnish officials suspected Moscow.
“The impression that someone is organizing and regulating things on the Russian side is probably true,” Finland’s foreign minister, Timo Soini, told the country’s state broadcaster at the time. “It is quite obvious that activity like this is a managed effort.”
Other E.U. members also have accused Russia of directing migrants into European countries. Poland has said that Belarus, a key Russian ally, lured migrants from the Middle East and Africa with flights and visas, and then pushed them into Poland to destabilize the country and gain diplomatic leverage. Warsaw said Moscow was behind the flow — allegations Moscow denied.
In recent days, Finland has again been sounding alarms about an increase in migrants at the border — relatively small numbers that it said were a significant change.
The Finnish Border Guard announced on Monday that it would no longer allow crossings by bicycle at three checkpoints over what it said was a “phenomenon of illegal entry” that saw people “without adequate travel documents” trying to enter.
“Apparently they got the bikes near the border,” Jukka Lukkari, the deputy commander of the southeast border guard, told Finland’s national broadcaster, Yle, on Tuesday. “They ride quite a short distance.”
The next day, Finland’s interior ministry warned that it was considering closing the crossings with Russia.
“The total number of asylum seekers at the eastern border is still relatively low, but the number has grown sharply in a short period of time,” it said in a statement on Wednesday, without providing numbers. The ministry attributed the increase to a “change” in Russia, saying that the authorities there had started allowing people to travel to Finland “without the proper documents.”
Maria V. Zakharova, the spokeswoman for Russia’s foreign ministry, called the allegations “unsubstantiated” and dismissed them as “misinformation.”
The people attempting to cross are mainly from the Middle East and Africa, according to the Finnish border authorities.
Around 280 asylum seekers not holding the proper documents have arrived at Finland’s eastern border since September, Yle reported, citing the Finnish Border Guard. There were 75 asylum seekers recorded on Wednesday and 55 on Tuesday, according to data from the Border Guard.