Swedish military officials said Tuesday that they spotted four Russian military jets in international airspace near the Danish island of Bornholm and the Swedish island of Gotland at around 5.30am.
According to Swedish news agency TT, the Russian aircraft - two TU-22M planes and two SU-27 fighter jets - were not using transponders, the telecommunications devices that emit and gather signals from planes.
"It is the first time in years that we have seen precisely this type of aircraft over the Baltic Sea," Anders Grenstad, Deputy Director of Operations for the Swedish Armed Forces, told TT.
He said the planes were understood to have first flown over the Baltic coast, then turned south toward Bornholm and Gotland before heading back to the Finnish Gulf.
See also: Russia simulated an attack on Denmark
The spotting of the jets comes just days after tensions between Denmark and Russia were intensified over comments made by Moscow’s representative in Copenhagen, who warned that Denmark faced a potential nuclear attack for aligning itself with Nato’s missile defence system.
“I don’t think the Danes fully understand the consequences of what will happen if Denmark joins the American-controlled missile defence. If it happens, Danish war ships will become targets for Russian atomic missiles,” Russian Ambassador Mikhail Vanin wrote in an op-ed in Jyllands-Posten newspaper.
Over the past months, there has been increased Russian airspace activity over and near Denmark.
In December a Russian military jet nearly collided with a SAS passenger plane out of Copenhagen. An October report from the Danish Defence Intelligence Service (Forsvarets Efterretningstjeneste) revealed that Russia carried out a simulated attack on Bornholm last summer, and in March last year a Swedish SAS pilot said his plane was “just seconds” from crashing into a Russian jet.
Denmark has said it’s not overly concerned about Russian aggression and is ready to “do what it necessary”.
Denmark earlier this month announced closer military cooperation with Sweden, with Defence Minister Nicolai Wammen saying that Russian activity played a role in the neighbours’ new partnership.
“Of course the latest developments in the east play a part and have given us a reason to have a look at areas where we could extend our co-operation,” Wammen told Ritzau.