Russian warplanes have made several passes of a US destroyer sailing in the Black Sea, the Pentagon has said, adding that in no way has their vessel acted "aggressively".
The US Department of Defense said the ship, the USS Ross, was "well within international waters at all times, performing routine operations".
On Monday the US Navy released video of a Russian plane passing as close as 500m to the USS Ross, which is en-route to Rota, Spain. The vessel is undertaking a "routine patrol conducting naval operations with allies and partners" in order to advance "security and stability" in the Black Sea, it said. The mission had been publicly announced.
According to the BBC, Russian media have suggested the American vessel "acted provocatively and aggressively", something the US has denied.
The US Navy said: "Ross observed the routine flights of SU-24 aircraft and had no interaction while both operated in international waters.
"Ross continued on her mission after observing the aircraft return to base. At no time did Ross act aggressively nor did she deviate from her planned operations. The conduct of her crew has been and continues to be professional.
"The US Navy operates ships in the Black Sea on a routine basis, consistent with the Montreux Convention and International Law."
The plane was not armed with weapons, according to reports.
A military source quoted by Russian media added of the encounter with USS Ross: "Su-24 attack aircraft demonstrated to the American crew readiness to harshly prevent a violation of the frontier and to defend the interests of the country."
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The incident is the latest in a series of border surveillance encounters involving the Russian military in the Black Sea, amid continuing hostilities in eastern Ukraine. European nations have increasingly had to scramble jets amid fears Russian aircraft were entering their airspace.
The US operate several warships, including NATO vessels, in the Black Sea, close to Russia's naval bases.