A Russia Su-27 jet fighter flew dangerously close and nearly collided with a U.S. reconnaissance aircraft this week in the latest aerial provocation by Moscow, defense officials revealed to the Washington Free Beacon.

The Su-27 conducted the close-in intercept of an RC-135 reconnaissance aircraft in international airspace over the Baltic Sea on Tuesday, said officials. The incident prompted a diplomatic protest.

"On the morning of April 7th, a U.S. RC-135U flying a routine route in international airspace was intercepted by a Russian Su-27 Flanker in an unsafe and unprofessional manner," said Pentagon spokeswoman Eileen M. Lainez.

"The United States is raising this incident with Russia in the appropriate diplomatic and official channels," she said in a statement.

A defense official said the Russian fighter jet flew within 20 feet of the unarmed reconnaissance jet in what the official called a "reckless" encounter that endangered the lives of the RC-135 crew.

No details were available regarding the mission of the RC-135, which was in a position to monitor Russian military activities in western Russia and Kaliningrad.

In Moscow, a Russian Defense Ministry spokesman confirmed the incident.

Maj. Gen. Igor Konashenkov, the spokesman, said the intercept was carried out after the aircraft was detected by Russian radar.

"Russian air defense radars spotted an unidentified air target over the Baltic Sea making steady progress toward the national border," he said according to several state-controlled news outlets. The report said the U.S. aircraft was operating without its signal transponder turned on.

"No emergency situation was reported during the fly-by of the American reconnaissance aircraft," Konashenkov said.

The RC-135 is a militarized and upgraded Boeing 707 jetliner that can be configured for several types of intelligence gathering, including photo, nuclear monitoring, and electronic spying.

The RC-135U variant involved in Tuesday’s near collision is code-named Combat Sent and conducts technical intelligence gathering on enemy electronic signals and radar emitters.

The monitoring comes amid new worries that Russia is deploying new short-range Iskander nuclear capable missiles in Kaliningrad and Russian-occupied Crimea in the Ukraine.

A second defense official said there have been no recent Russian aerial provocations near U.S. coasts. But Moscow is expected to ramp up its training operations flights around this time of year.

"That means we’re probably due for [aerial encounters] soon," the official said.

The most recent similar encounter took place March 24 when two Su-27s, along with two nuclear capable Tu-22 Backfire bombers, conducted flights over the Baltic. The Russian jets were flying without signal beacon transponders that permit air traffic controllers to monitor their flight paths. They were intercepted by Swedish jets.

It could not be learned if U.S. or NATO jets were sent to escort the RC-135 over the Baltic Sea.

The threatening aerial encounter followed a series of provocative Russian military aircraft encounters, mainly involving the dispatch of nuclear-capable Tu-95 Bear bombers near U.S. and European coasts.

Flights of Russian strategic aircraft near U.S. and allied airspace have sharply increased as part of a campaign of nuclear saber rattling by Moscow.

Adm. William Gortney, commander of the U.S. Northern Command, expressed his military concerns about the increase in Russian military flights and provocations during a briefing with reporters the same day of the RC-135 incident over the Baltic.

"The Russians have developed a far more capable military than the quantitative, very large military that the Soviet Union had," Gortney said, adding that Moscow has adopted a new strategic doctrine that is being demonstrated by the provocations.

"At the same time, they are messaging us," he told reporters at the Pentagon. "They're messaging us that they're a global power—we do the same sort of thing—with their long-range aviation."

Gortney said the numbers of incidents have gone up but he did not have the percentages.

"And so we watch very carefully what they're doing," he said. The Russians need to adhere to "international standards that are required by all airplanes that are out there," he said, "and everybody is flying in a professional manner on their side and our side as we watch very closely."

Eric Edelman, former undersecretary of defense for policy, said the latest incident appears to be part of a pattern of activities by Russia that began around 2007 when Russian President Vladimir Putin began protesting U.S. missile defenses in Europe. The provocative activities have taken place in both the skies and on the sea, Edelman said.

The Russians are engaged in what Edelman said is "station identification"—signaling that they remain a relevant nuclear weapons power.

"It’s part of a pattern now of very, very provocative activities, both in the air and on the sea," Edelman said in an interview.

The Russians are signaling that "we’re still here, we’re still an important military power, your nuclear peer, and they are seeking to intimidate the Balts, Swedes, and Finns," he said. The Baltic states are Latvia, Lithuania, and Estonia.

A report by the European Leadership Network, "Dangerous Brinksmanship: Close Military Encounters Between Russia and the West in 2014," states that last year NATO aircraft conducted more than 100 intercepts of Russian aircraft, three times the number of intercepts in 2013. A total of 11 encounters were described as being of a serious and "more aggressive or unusually provocative nature, bringing a higher level risk of escalation."

"These include harassment of reconnaissance planes, close overflights over warships, and Russian ‘mock bombing raid’ missions," the report said, noting that the intensity and gravity of the incidents coincided with the Russian annexation of Crimea.

"These events add up to a highly disturbing picture of violations of national airspace, emergency scrambles, narrowly avoided mid-air collisions, close encounters at sea, simulated attack runs, and other dangerous actions happening on a regular basis over a very wide geographical area."

The report said the Russians appear to be testing NATO and European defenses, as well as using the provocative actions to contribute to an information warfare campaign.

The Russian provocations "serve as a demonstration of Russia’s capability to effectively use force for intimidation and coercion, particularly against its immediate neighbors," the report said.

Brian McKeon, principal deputy undersecretary of defense for policy, said in Senate testimony in February that Russian nuclear actions are a significant problem.

"Russia’s recent behavior currently poses one of our most pressing and evolving strategic challenges—challenges felt across the strategic forces mission space," McKeon said.

"We are confronted with Russia’s occupation of Crimea, continuing Russian aggression in eastern Ukraine, Russia’s increasingly aggressive nuclear posturing and threats, including the prospect of nuclear weapons in Crimea, and its violation of the Intermediate-Range Nuclear Forces (INF) Treaty."

Adm. Cecil Haney, commander of the U.S. Strategic Command, stated in testimony to the Senate in February that Russian aerial provocations were part of a number of "troubling actions" by Moscow

Until recently, military spokesmen have sought to play down the Russian aerial provocations, frequently dismissing intrusions into U.S. and Canadian air defense identification zones as not a threat.

"It’s ‘station identification’ and a former of intimidation, and it’s dangerous," said Edelman, a former ambassador to Finland. "Some time something bad is going to happen, particularly against the backdrop of what’s going on in the Ukraine, and it could lead to inadvertent escalation and confrontation. It’s very dangerous."

UPDATE Saturday, April 11, 11:10 A.M.: This article has been updated with comment from a spokesman for the Russian government, who confirmed the incident.