BERLIN — In the latest in a string of dangerous encounters between Russian and NATO military jets, a U.S. Navy plane came within five feet of a Russian Su-27 jet on Monday, according to the State Department. The incident occurred in international airspace above the Black Sea, raising renewed concerns over the risk of midair collisions in Europe.

State Department spokeswoman Heather Nauert said the United States considers the incident to be of “the highest level of concern,” calling it only “the latest example of Russian military activities disregarding international norms and agreements.”

Provocative Russian behavior, especially since 2014, has set off alarm bells across Europe over the past few years.

Last June, a Russian fighter jet similarly came within a few feet of a U.S. reconnaissance plane over the Baltic Sea. It then lingered by the side of the U.S. plane for minutes, U.S. military officials said at the time.

report issued in 2014 by the European Leadership Network, a London-based think tank, documented almost 40 incidents that together “add up to a highly disturbing picture of violations of national airspace, emergency scrambles, narrowly avoided midair collisions, close encounters at sea, simulated attack runs, and other dangerous actions happening on a regular basis over a very wide geographical area,” according to the report. The report was only compiled for 2014 and not for the subsequent years.

At the time, the think tank reached the conclusion that Russia was not actively trying to provoke a full-blown conflict with NATO but that its actions nevertheless “could prove catastrophic” because of the risk of unintended escalation. Russian officials have rejected those accusations, with former Russian Ambassador to the United States Sergey Kislyak saying the unusual frequency in dangerous encounters was the result of an increase in military training flights.

But in the worst case, some of the documented incidents may have resulted in mass-casualty accidents. On March 3, 2014, the pilots of a Russian reconnaissance aircraft — which apparently had not transmitted its position — nearly collided with a passenger plane that had just taken off from Copenhagen, according to the European Leadership Network. The pilots of the civilian SAS 737 plane with 132 passengers aboard managed to avoid a collision, thanks to good visibility.

Unlike Monday’s dangerous encounter above the Black Sea, most of 2014’s reported incidents occurred around the Baltic Sea and the North Sea.

Whereas some of the documented incidents have involved U.S. planes, European aircraft still recorded the most encounters. In mid-January, British Air Force jets scrambled to intercept Russian bombers near the country’s airspace amid growing concerns of Russian activity near Britain.

Over Christmas, British ships and a helicopter were also dispatched to track what officials said was an “upsurge” of Russian naval vessels passing near its waters.

Russian submarines have also stepped up activity around undersea data cables in the North Atlantic, according to my colleague Michael Birnbaum. The cables, which NATO officials fear could be targeted by Russian submarines, provide Internet and other communications connections to Europe and North America, representing vital data lines.

The Russian submarine activity — unprecedented since the Cold War — has sparked NATO hunts, involving a number of U.S. Navy tracking flights. If anything, the airspace above Europe only appears to be getting busier.

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