Sweden's government says it will summon Russian officials to protest after four fighter jets violated its air space.
Stockholm says that four Russian aircraft briefly flew over Swedish airspace on Wednesday, east of the island of Gotland in the Baltic Sea.
The jets in question were two SU-27 and two SU-24 fighters, according to the Swedish Armed Forces, adding that they had "total control" of the situation.
“In light of the current situation we are very concerned about the incident,” Swedish Air Force chief Carl-Johan Edstrom said.
“This is unprofessional and irresponsible behaviour from the Russian side.”
Swedish fighter jets were scrambled and took photos of the Russian jets, the statement added.
On Thursday, a spokeswoman for the Swedish foreign ministry told AFP that it would summon Russian diplomats over the incident, which took place amid the war in Ukraine.
"There are established procedures for such matters and they involve summoning a representative of the nation responsible for the violation to the foreign ministry," Klara Hook said.
On Sunday, Sweden announced that it would send 5,000 anti-tank weapons and other military equipment to Ukraine, the first time it has exported arms to a country at war since 1939.
Both Sweden and neighbouring Finland have seen renewed debate over joining NATO following Russia's invasion of Ukraine.