For what NATO’s secretary-general described as a "long time" and Russia’s Defense Ministry characterized as just "a few seconds," a Russian fighter jet violated Turkey’s airspace on Saturday, increasing tensions and raising concerns about Russian aggression beyond mere saber rattling.

NATO said that a Russian aircraft also violated Turkish airspace on Sunday. The response was swift from Western powers that were already anxious about Russian warplanes flying near their own aircraft in Syria and near their own sovereign airspace.

Igor Konashenkov, a spokesman for the Russian Defense Ministry, said that a pilot flew into Turkish airspace on Saturday by mistake, because "of unfavorable weather conditions in that area," according to Tass, Russia’s government-owned news agency.

But Jens Stoltenberg, the NATO secretary-general, said "it doesn’t look like an accident," and there have been a number of other instances recently of Russian military flights entering the airspace of NATO members or coming very close, prompting interception by Western military aircraft.

The presumed purpose of such incursions is to gauge the target country’s reactions. Which radar systems and which missile batteries are activated and reveal themselves? Which planes are scrambled from which bases, and how quickly? How loudly and publicly do leaders complain, and how much backing do they get at home and from international allies?

In February, as tensions mounted in Ukraine, the Royal Air Force scrambled warplanes to intercept Russian bombers flying off the coast of Cornwall in southwest England. The incident carried echoes of the Cold War, when Soviet aircraft would routinely test Western defenses in this way, and NATO would do the same to the Soviet bloc.

The February encounter was the closest to occur near England in decades. British researchers noted at the time, citing NATO officials, that the number of Russian aircraft intercepted as they approached Western airspace had tripled from 2013.

In 2014, Norway scrambled F-16 fighters to intercept Russian warplanes off its coast 74 times, a 27 percent increase from 2013. The Russians did not actually violate Norwegian airspace, but they have done so in the Baltic nations, which were once part of the Soviet Union.

In October 2014, NATO reported an "unusual" spike in Russian military flights across Europe, according to The Associated Press. The alliance observed large-scale maneuvers in international airspace involving Tu-95 Bear H strategic bombers, MiG-31 fighters and other Russian warplanes.

Norway, Britain, Portugal, Germany, Denmark and Turkey scrambled fighters in response; so did two non-NATO countries, Finland and Sweden, according to The AP.

Russia’s involvement in Syria is the first time in a generation that it has conducted military operations outside the former borders of the Soviet Union. Though it maintains that its airstrikes target the Islamic State militant group, just as the U.S.-led coalition’s airstrikes do, Western officials say Russian forces have actually been attacking other rebel groups instead, including some that receive assistance from the United States.

Though the Russians have so far mounted most of their airstrikes far away from areas where the U.S.-led coalition mainly operates, the skies over Syria are now crowded with military aircraft. The risk of accidents and unplanned confrontations has risen, prompting calls from Western countries for greater coordination between the United States and Russia.