A Russian fighter jet flew dangerously close to an American reconnaissance aircraft during an "unprofessional" intercept in international airspace over the Black Sea last month, the Pentagon said Friday.

The American RC-135 was conducting routine operations May 30 when a Russian SU-27 Flanker flew up from behind, passed underneath then crossed over to the American plane's right-hand side, spokesman Colonel Steve Warren said.

"We don't know exactly why this intercept was conducted unprofessionally," he told reporters.

"It may have been a lack of training on the part of the Russian pilot. It may have been just a general level of unprofessionalism of the individual who was flying the Flanker."

CNN reported that the Russian plane came within just 10 feet of the US aircraft. Warren said only that the Russian jet passed at a "distance we felt was unprofessionally and unsafely close."

The US plane was intercepted a second time on the same flight, but that was deemed to have been done appropriately, Warren said.

Russian and US planes routinely monitor each other's movements in international airspace, but this encounter is the latest in a series of incidents sparking concerns over how Russia carries out airborne intercepts.

On April 7, a Russian Flanker jet performed "aggressive manuevers" in close proximity to a US reconnaissance plane in an incident that occurred over the Baltic Sea, in international airspace north of Poland.

The US Navy last month posted a video on YouTube showing a Russian SU-24 jet flying close to the American guided-missile destroyer USS Ross in the Black Sea. The Navy said the Russian jet was not carrying weapons.

NATO jets have in recent months been scrambled to intercept Russian planes approaching Baltic countries' airspace, amid tensions over the crisis in Ukraine.