The Pentagon has said most interactions between Russian and American warplanes go smoothly, including in a case a year ago in which a Russian jet came within 20 feet of a Navy P-8 surveillance plane. Ms. Nauert called it "the latest example of Russian military activities disregarding global norms and agreements".
The United States condemned "unsafe Russian military practices" after a Russian Su-27 fighter flew within 5 feet of a US surveillance airplane over the Black Sea on Monday.
On Monday evening, the U.S. State Department issued a statement, noting the incident with the "highest level of concern".
CNN military analyst John Kirby blasted the SU-27's actions as "inexcusable, provocative and potentially fatal".
There have been a number of aerial intercept incidents between U.S. and Russian aircraft over the past year, even outside of Europe.
"While the USA aircraft was operating under global law, the Russian side was flagrantly violating existing agreements and worldwide law", Nauert said.More news: House of Cards announces Kevin Spacey replacements for final season
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The Russian defense ministry said the Su-27 returned to its base after the American plane changed its course, according to Reuters, citing the Russian news agency RIA.
The encounter involved a Russian SU-27 fighter jet and a U.S. EP-3 Aries aircraft.
Russian, US and North Atlantic Treaty Organisation forces operate in close proximity to one another in the area, particularly since Russia boosted its military presence in the region following its annexation of Crimea in 2014, CNN reported.
According to the navy the jet came within five feet of the plane. As a result, the aircraft experienced "a 15-degree roll and violent turbulence".
Russian and USA aircraft have had unsafe encounters over the Black Sea before. The US Aircraft had to fire flares as warnings to the Russian jets, one of which "had to aggressively maneuver to avoid a midair collision".
"The crew of the fighter jet reported the identification of the American reconnaissance aircraft and accompanied it, preventing it from violating Russian airspace, observing all necessary security measures".