On May 9, 2017, a Russian SU-27 came within approximately 20 feet of a U.S. Navy P-8A Poseidon while the U.S. Navy aircraft was conducting routine operations in international airspace, according to Captain Pamela Kunze, a spokeswoman for U.S. Naval Forces Europe, NBC News reported.

She added that the interaction was considered safe and professional by the P-8A's mission commander.

Read alsoNATO intercepts 780 Russian military planes in 2016U.S. Navy aircraft and ships routinely interact with Russian ships and aircraft in international seas and water and most interactions are safe and professional, she said.

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Twenty feet is extremely close and other encounters at much greater distance have been deemed unsafe and unprofessional by the U.S. military in the past. Kunze said that distance is only one of many variables considered when defining what is safe and professional.

Read alsoRussian bombers fly near Alaska, U.S. Air Force scrambles jets – mediaShe cited speed, altitude, rate of closure, visibility and other factors that impact whether an incident is characterized as safe or unsafe, professional or not professional.

It is up to the commander of the ship or aircraft to evaluate all of the variables and assess each interaction individually.