(Bloomberg) -- Russia “may be testing” the NATO member countries on its western frontier with recent actions linked to demarcating its borders, Finnish Prime Minister Petteri Orpo said.

Speaking in Riga on Friday during his visit to Latvia — another Russian neighbor that’s part of the North Atlantic Treaty Organization and the European Union — Orpo said “we have to be well prepared against all kinds of threats coming from Russia.” 

His comments follow an incident on Thursday where Russian border authorities were accused by their Estonian counterparts of removing a set of buoy markers on the Narva River separating the two countries. Earlier this week, a Russian Defense Ministry proposal to modify the country’s Baltic Sea border and territorial waters prompted vocal responses from Lithuania and Finland. The document outlining the draft proposal was later removed from a government website without explanation.

Read More: Estonia Says Russia Removed Border Markers as Tensions Rise 

“Of course we can see that there are time connections with all these issues in Finland, in Lithuania, in Estonia but now it’s time to be calm and look very carefully at what really is happening,” Orpo said. “Maybe they are testing us, but we don’t know yet.” Russia will remain “the most significant direct threat to European security, he added.” 

The European Union “expects an explanation by Russia about the removal of the buoys, and their immediate return,” the bloc’s External Action Service said in a separate statement on Friday, adding Russia’s actions are “unacceptable.”

Orpo’s Latvian counterpart Evika Silina, speaking at the same news conference, said “it is clear that Russia is seeking new ways to use hybrid actions in the sea as well as on land borders,” adding Russia’s neighbors “have to be resilient.”

(Updates with European Union comment in fifth paragraph.)

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