Helsinki: Sweden says it will summon Russia's ambassador on Monday to explain why a Russian fighter jet flew just 20 metres away from a Swedish reconnaissance plane in international airspace over the Baltic Sea earlier this week.

Foreign Ministry spokeswoman Diana Qudhaib told Swedish public broadcaster SVT on Sunday that Sweden considers Tuesday's incident serious, saying the Russian Sukhoi Su-27 plane acted in "an improper and unprofessional manner in a way that jeopardised flight safety."

The Swedish Gulfstream aircraft was flying on a routine surveillance mission when the Russian jet passed by at a very close distance. The Swedish military published a photo of the incident.

In January, a Russian reconnaissance plane escorted by two Su-27 jets briefly violated Sweden's airspace.

AP