Denmark’s foreign minister Lars Lokke Rasmussen summoned Russia’s ambassador Vladimir Barbin on Tuesday after a Russian frigate fired two flares at a Danish air force helicopter on Monday. One flare passed close to the helicopter during routine photographic coverage in international waters off southern Denmark. Rasmussen called the act “unacceptable,” while Prime Minister Mette Frederiksen labeled it “reckless action” meant to intimidate and divide NATO allies supporting Ukraine.

The incident follows a string of Russian airspace incursions, drone violations, and sabotage plots across Europe. On Monday, a Russian strike hit a Ukrainian train station near the Polish border where former C.I.A. director David Petraeus, former British prime minister Boris Johnson, and other officials had been traveling. Lithuania’s President Gitanas Nauseda said NATO jets destroyed a drone entering Lithuanian airspace overnight, and Poland’s defense minister Wladyslaw Kosiniak-Kamysz reported recovering a drone from the Baltic Sea likely from Russia.

Barbin said Russia would investigate but accused Denmark of dangerous maneuvers near Russian warships. European leaders say Russia is escalating provocations as the war in Ukraine enters another winter.