Finland will leave just one border crossing open on its frontier with Russia to stem a flow of asylum seekers seen as a hybrid operation masterminded by Moscow.

The Nordic country will close another three road checkpoints after already shuttering the four busiest ones last week, Prime Minister Petteri Orpo said on Wednesday, citing a “worsening situation.”

Only the northernmost crossing, Raja-Jooseppi in Lapland, will stay open starting in the early hours of Nov. 25, Interior Minister Mari Rantanen told reporters in Helsinki. The decision is “unavoidable,” she said, adding the government stands ready to do more if needed.

Orpo had on Monday warned Russia that more border crossings would be closed if migrants continue to be brought there by Russian officials.

Asylum seekers from countries including Yemen and Afghanistan have continued to arrive at Salla and Vartius check points, which are located in remote areas. Many come lightly clothed in freezing weather, most with bicycles, including children’s bikes, local media have reported. Their lack of gear and suitable vehicles indicates they are getting transport from Russia.

Finland’s government says the operation threatens national security, and on Saturday sealed the southeastern stretch of the border in response.

Read More: Finland Closes Russian Border Crossings Citing Security Risk

The newest member of the North Atlantic Treaty Organization guards 1,300 kilometers (800 miles) of border with Russia, the alliance’s longest stretch facing its main adversary.

Finland updated its border guard laws last year to give authorities a toolkit to counter hybrid influence activities, including an arrival of exceptionally high number of immigrants in a short period of time or what they termed the “instrumentalization of migration” by a state.

That’s after experiencing a similar operation in 2015 to 2016. Poland and Lithuania are among European Union members to have encountered the phenomenon.