The guided-missile destroyer USS Donald Cook reported that pairs of Russian Su-24 attack planes made numerous close-range passes on Monday and Tuesday.
The planes appeared to be unarmed. On at least one occasion, an Su-24 came within an estimated 30 feet (9 metres) of the Cook, which was in international waters about 70 nautical miles from the Russian enclave of Kaliningrad. The Cook did not respond except to unsuccessfully query the Russian pilots by radio.
In remarks to CNN Espanol, Kerry condemned the Russian actions and said that "under the rules of engagement" it "could have been a shoot-down." In all cases, a military commander has the authority to defend his or her ship, plane or other unit.
The commander is expected to use his or her best judgement to determine whether the ship faces an imminent threat. White House spokesman Josh Earnest said the US has raised its concerns through its military defence representative at the US Embassy in Moscow.