Bulgaria has claimed a rise in violations of its airspace by Russian military and commercial aircraft, calling it provocative.
According to Defense Minister Nikolay Nenchev, Russian aircraft had entered "Bulgaria's area of responsibility" in NATO airspace four times in the past month.
"It is very worrying, so we take preventive measures," scrambling Bulgarian fighter jets, he told Nova TV channel on Sunday.
During their violations, Nenchev said, Russian planes had turned off their communications devices that make it easier for an airplane to be located.
Bulgaria, he said, has demanded explanation from Russia over the alleged violations.
The Balkan country, once a staunch ally of Moscow, has inched closer to the Western bloc, becoming a NATO member in 2004.
Bulgaria has expressed readiness to take part in a multinational NATO land brigade based in Romania.
However, it would not join a proposed NATO fleet meant to counter Russian forces in the Black Sea as it did not want a war there, Bulgarian Prime Minister Boiko Borisov said last month.
The East European country is almost entirely dependent on Russian energy supplies, and many Bulgarians feel a deep affinity with their giant neighbor across the Black Sea.
Tensions between Russia and NATO have risen, particularly since the Ukraine crisis. NATO has said it plans its biggest build-up in the region since the Cold War.
Russia is reinforcing its western and southern flanks with three new divisions in response to the build-up, including readying of US missile systems in Romania, Poland and the Baltics.
The US has long insisted that the system is directed against Iran, but Russian officials have slammed the move as an "attempt to destroy the strategic balance" in Europe.
Russia has described the US missile system in Europe as a "threat" and says it is taking "protective measures" to guard against it.
In recent months, the two sides have repeatedly accused each other of staging erratic and aggressive maneuvers by their warships and aircraft in flash point areas, mostly in the Baltic Sea.