RAF jets were scrambled to intercept Russian aircraft approaching the Baltic skies, according to Defence Secretary Michael Fallon.
Mr Fallon described the incident as an "act of Russian aggression".
The military transport aircraft did not transmit a recognised identification code and appeared to be unresponsive.
The interception was the first such intervention as part of the RAF's latest mission to keep airspace in the area secure as part of the Baltic Air Policing Mission.
Four RAF jets were deployed to join the mission in the Baltics in April and will remain there until the end of August.
The initiative sees Nato members help others who do not have their own policing capabilities.
One of the pilots involved in the mission said it went "as planned".
During the last mission Typhoon fighter jets were scrambled 17 times, intercepting more than 40 Russian aircraft.