The U.S. is protesting an intercept of a U.S. reconnaissance plane by a Russian fighter jet last week, calling it "unsafe and unprofessional" amid what it views as increasingly aggressive air operations by Moscow.

Pentagon spokesman Mark Wright on Sunday said the U.S. was filing a complaint to Russia after the April 7 incident over the Baltic Sea.

Russian officials have denied their pilot did anything wrong, according to several news reports.

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According to the Pentagon, the U.S. RC-135U plane was flying in international airspace north of Poland. U.S. officials say a Russian SU-27 fighter intercepted the U.S. aircraft at a high rate of speed from the rear, and then proceeded to conduct two more passes using "unsafe and unprofessional manoeuvrs" in close proximity.

"Unprofessional air intercepts have the potential to cause harm to all aircrews involved. More importantly, the careless actions of a single pilot have the potential to escalate tensions between countries," Wright said.

"This air activity takes place in the context of a changed security environment in view of Russia's aggression against Ukraine," he said.

It isn't the first time the U.S. has protested to Moscow what it considered to be an unsafe intercept. Last April, a Russian fighter jet intercepted a U.S. reconnaissance plane in international airspace over the Sea of Okhotsk.