A Russian fighter jet SU-27 flies over the sea off the Japanese northern island of Hokkaido, in this handout photo taken February 7, 2013 by Japan Air Self-Defence Force.

A Russian fighter jet came within 10 feet of an American anti-submarine plane in the Black Sea on Wednesday, stoking tensions in the disputed waters just south of Crimea.

Unnamed US defense officials told Reuters on Wednesday that the Russian Sukhoi SU-27 fighter jet had intercepted an American P-8 Poseidon surveillance plane in the Black Sea.

The officials said that, at one point during the 19 minute interception, the planes came within 10 feet of each other. One Pentagon source called that "unsafe and unprofessional."

Russian military sources fired back, with one general accusing the American plane of flying dark.

"On September 7, the US P-8 Poseidon surveillance airplanes tried to approach the Russian border twice," said Major General Igor Konashenkov in a statement, according to state-owned English-language broadcaster RT. He added that the planes were flying "with their transponders off."

A search through FlightRadar24.com, a website that tracks civilian and military aircraft via their transponders — and which has, in the past, picked up military surveillance aircraft — shows neither the P-8 nor the SU-27 over the Black Sea on Wednesday morning, when the Russian military says the interception occurred.

There's nothing necessarily unusual about the American plane flying dark. The P-8 has become the go-to American plane for maritime surveillance and sub hunting. Its deployment over the Black Sea, which has come amid increasing Russian submarine activity in the area, had not been public knowledge prior to Wednesday.

Russia's claim that the American plane was approaching Russian airspace is quite possibly true, if you accept that Russia's territorial claim to the Black Sea waters off the coast of Crimea. That's something that the White House, obviously, does not.

The American defense officials who spoke to Reuters contended that the US aircraft had been flying legally over international waters. Russia, however, considers a sizeable chunk of the Black Sea as its territory — even though its annexation of Crimea was met with international condemnation — and has consistently engaged in oil and gas exploration off the coast of Crimea.

On top of submarine exercises, Russia has been running extensive training in the Black Sea, which includes minesweepers, missile cruisers, and — perhaps not coincidently — simulated aircraft interceptions.

NATO, meanwhile, has been running extensive training exercises throughout Eastern Europe, and in Ukraine itself.

It's not the first time that the P-8 has been intercepted, however. In 2014, a Chinese aircraft intercepted the P-8 over the Pacific Ocean, coming within 20 feet of the American plane.

But Russian interceptions of American aircraft are happening with increasing frequency. Russia has intercepted or buzzed (when a helicopter or plane flies directly over a ship) NATO planes and vessels dozens of times in recent years.