Swedish officials on Wednesday accused four Russian fighter jets of violating its airspace over the Baltic Sea.
"With the current situation as backdrop, we take this incident very seriously. Russia's conduct is unprofessional and irresponsible," Swedish Air Force chief Carl-Johan Edstrom said in a statement.
Sweden's military scrambled its jets, which took photos of the Russian planes, according to the statement.
"This demonstrates that our readiness is good," Edstrom said. "We were on-site to secure the territorial integrity and Sweden's borders. We have complete control of the situation."
Other details about the incident were not made available. Russia has not publicly commented on the alleged airspace violation over Swedish territory.
"The Russian violation of Swedish airspace is of course completely unacceptable," Defence Minister Peter Hultqvist told the TT news outlet. "It will lead to a firm diplomatic response from Sweden. Swedish sovereignty and territory must always be respected."
Several days before, Swedish officials confirmed they would send military aid, including 5,000 anti-tank weapons to Ukraine. It's the first time that Sweden has sent weapons to a country at war since 1939—when Sweden sent weapons to Finland after the Soviet Union invaded during the Winter War, analysts noted.
Finland also said it would send military rifles and ammunition to Ukraine's military, according to reports.
Both Sweden and Finland, which are members of the European Union, do not belong to NATO, although they are close NATO partners. Since Russia began its invasion of Ukraine in late February, several Swedish politicians publicly mused joining the security organization, which would likely further fray ties with Moscow.
"Anything is possible at the moment and the signal from NATO countries is that a membership application can be processed in a very short time-span," Zebulon Carlander, defense analyst with the Society and Defense organization in Sweden, told the AFP news agency this week.
However, Russia's Foreign Ministry last week said that Sweden and Finland could face "serious military and political repercussions" should they join NATO.