Two Russian planes flew close to a US guided missile destroyer almost a dozen times, American officials have said.
The Sukhoi SU-24 warplanes, in international waters in the Baltic Sea, had no visible weaponry and the ship took no action.
One official called the events on Monday and Tuesday "one of the most aggressive acts in recent memory".
The commander of the missile destroyer, the USS Donald Cook, described the flights as a "simulated attack".
The passes were "unsafe, potentially provocative" and "could have caused an accident," officials told the Wall Street Journal.
At one point the jets were so close, about 30ft (9m), that they created wakes in the water around the ship.
The actions may have violated a 1970s agreement meant to prevent dangerous incidents at sea, but it is not clear whether the US is going to protest.
A Russian helicopter taking pictures also passed by the ship seven times.