For the second time this year, a Russian military aircraft turned off its transponders to avoid commercial radar and nearly collided with a passenger jet over Sweden, officials have said.

Swedish authorities said that on Friday, a Russian military aircraft nearly collided above southern Sweden with a commercial passenger jet that had taken off from Copenhagen in Denmark.

Sweden’s air force chief, Major General Micael Bydén, said the aircraft’s transponders, which make the plane visible to commercial radar, were shut off. Swedish fighter jets were sent up to identify the aircraft, and Hultqvist later identified it as a Russian intelligence plane.

“This is serious. This is inappropriate. This is outright dangerous when you turn off the transponder,” Swedish defence minister Peter Hultqvist said on Swedish radio.

Officials at Russia’s ministry of defence in Moscow were not available to comment on Saturday.

In recent months, Russia has increased its military presence in the Baltic Sea area, prompting some Swedish officials to compare it to the Cold War. In October, non-Nato member Sweden launched its first submarine hunt since the collapse of the Soviet Union. Swedish authorities said a small, foreign submarine had entered its waters illegally but never found it and did not disclose its nationality.

Nato has air patrols over the Baltic Sea and the continuous rotation of Nato military units in and out of countries such as the Baltic states and Poland.

Bydén said the incident in international air space was fairly serious, adding the southern-bound commercial flight was immediately ordered to change course. Media in Sweden and Denmark said the commercial plane was en route to Poland, but no one identified the airline that was flying the jet or how many people it was carrying.

Byden said this was not as serious as in March when a Russian plane flying without transponders came within 100 metres of an SAS plane that had taken off from Copenhagen.