The incident, which comes as the Ukraine war has sharply heightened regional tensions, evokes the "classic provocative behaviour that we know from the time of the Cold War", said Defence Minister Boris Pistorius.

AFP understands the incident last week involved the crew of a Russian cargo ship firing red signal flares at a German helicopter without hitting it.

Pistorius did not go into detail about the incident but said "in this case no one was hurt and no one was in danger".

"We have repeatedly had incidents in the Baltic where it turned out that Russian ships were firing warning shots into the air or into the water," Pistorius told public broadcaster Deutschlandfunk.

While Germany "can never exclude" the possibility that such incidents would lead to direct escalation, Pistorius said that German vessels "and those of our allies are behaving prudently and with foresight".

"They report these incidents, they respond with de-escalatory measures and don't let themselves get taken in" by such behaviour, he said.

Worries about sabotage in the Baltic Sea were recently heightened after undersea data cables were cut, with suspicion falling on a Chinese vessel.

Foreign Minister Annalena Baerbock also hinted at the most recent incident in comments on Wednesday in which she referred to "helicopters from Germany suddenly being fired upon by tankers".

Germany is Ukraine's second-biggest supplier of military aid and German officials have repeatedly raised the alarm in recent months about Russian "hybrid warfare", including acts of sabotage.