A Russian jet has flown by a US spy plane over the Black Sea in an “unsafe and unprofessional manner,” coming within 15 feet (4.5 meters) from it, says the Pentagon.

"On January 25, 2016, a US RC-135U flying a routine route in international airspace over the Black Sea was intercepted by a Russian Su-27 in an unsafe and unprofessional manner," said Pentagon spokeswoman Army Lieutenant Colonel Michelle L. Baldanza.

Baldanza added that aircraft intercepts are routine events but “the Russian pilot acted in an unprofessional manner that put both the American flight crew and himself at risk."

A US RC-135 spy plane

One official said that the jet flew along the right side of the US plane, then veered off in a maneuver that had an adverse effect on the "controllability of the US aircraft."

The official said the incident occurred more than 40 miles from the Russian coast. However, he could not confirm if the Russian aircraft was armed at the time of the encounter.

Such close encounters over the Black Sea are not unprecedented as two similar incidents took place involving the same aircraft in 2014.

In one case, a Russian Su-27 flew within 100 feet of the nose of the US Air Force RC-135U over the Sea of Okhotsk between Russia and Japan, a move described by a US official as "one of the most dangerous close passes in decades."

During the encounter, the Russian jet "showed its belly" to the American crew so they could see it was armed with missiles.

Last year, on April 7, another Su-27 flew within 20 feet of an RC-135, this time over the Baltic Sea.

In a few occasions, Russian jets have also warned US warships operating in the waters, forcing them to change course.

The Pentagon says US military operations in the Black Sea are routine and in accordance with international law."

Russian Deputy Defense Minister Anatoly Antonov has dismissed the claim, saying US military operations near Russian borders pose a danger to strategic stability in the area.