Baltic state and NATO member Estonia has said a Russian military craft violated its airspace, the fourth such incursion this year, prompting the foreign ministry to hand a protest note over to the Russian ambassador in Tallinn, the army said Tuesday.
According to the army statement, a Russian Antonov An-72 transport aircraft entered Estonian airspace near the Baltic Sea island of Vaindloo without permission and remained there for one and a half minutes.
The Russian pilot had the aircraft's transponder turned on but was not in contact with Estonian air-traffic control, the statement said.
Citing Russian military experts, Postimees newspaper reported on Tuesday that Russian warplanes are not fitted with the most modern transponders, effectively meaning that civilian radar is "blind" to them.
Instead they use soviet-era encoded electronic transmitters that only transmit data to Russian military radar.
NATO and Russia have traded accusations that each side flies warplanes over the Baltic region with the transponder - an automatic transmitter that identifies an aircraft and its position - switched off.
Russia's Defence Ministry denied that its plane violated Estonian airspace, according to state news agency TASS.
"Russia's Aerospace Forces have not violated Estonia's airspace in the vicinity of Vaindloo Island," General Igor Konashenkov told journalists. An-72 warplanes of Russia's Aerospace Forces "did not make any flights in the indicated area yesterday."