Top EU diplomat Kaja Kallas sounded the alarm for Europe on Wednesday after Warsaw accused Russian-backed operatives of carrying out an explosion targeting a Polish railway.

“It is clear that these kinds of attacks are an extreme danger also for our critical infrastructure,” Kallas told journalists in Brussels on Wednesday.

“We have to have a strong response because what Russia is trying to do is two things. On one hand to test us, to see how far they can go … And next they also try to sow fear within our society,” she said.

Kallas was speaking hours after Poland said it was shutting down Russia’s last consulate in the country due to the railway sabotage, which Polish Prime Minister Donald Tusk said had been executed by Ukrainians working for Russia.

The Polish incident is just the latest in a string of so-called hybrid attacks to hit European countries in recent weeks, from airspace violations by Russian warplanes to drone disruption at airports across the continent to cyber attacks and acts of disruptive vandalism.

EU countries are debating how to respond to such attacks, with some leaders calling for a more robust response clearly attributing the attacks to Russia while others warn against coming out too strongly and spooking the public.

“Now our response is also dependent on those two factors,” added Kallas. “They want to sow fear inside our societies … if our response is too strong then the fear increases, which is what Russia wants. So we really have to have a balanced approach,” she said.

She added that Europe should “send a message of unity to Russia that they cannot get away with these attacks but at the same time give assurances to our society that there is nothing to be afraid of.”

Her message echoed what Finnish President Alexander Stubb told POLITICO earlier this week: “My recommendation is to stay calm. Have a little bit more sisu [grit]. Don’t get too flustered.”