The number of Russia’s long-range strategic bombers intruding the U.S. airspace doubled in 2014, forcing the U.S. Air Force to scramble American fighter jets more frequently. The increased activity of Russian bomber flights is considered by many as Kremlin’s warning to U.S. President Barack Obama not to intervene in its affairs related to Ukraine, the Washington Times reported.

The Russian strategic bombers buzzed the U.S. Air Defense Identification Zone (ADIZ) at least 10 times in 2014, which is twice the average of five invasions a year dating to 2006, according to the North American Aerospace Defense Command (NORAD). Although the U.S. does not claim sovereignty over the zone, it closely monitors and requests information of all planes entering the area. Any aircraft entering without permission may be identified as an enemy aircraft, which could lead to interception by fighter aircraft.

“We saw last year both an increase in their frequency as well an expansion of the areas where they flew. While these flights are ostensibly for training, they are also clearly intended to message to us,” Adm. William Gortney, the commander of NORAD, told the Washington Times.

According to Michael K. Conaway, R-Texas, Russia wants to test the U.S. response by its “provocative” actions.

“So why do the Russians continue to do it? I think they’re rattling sabers in a sense. I think this is definitely coming from [Russian President Vladimir] Putin. They’re flexing their muscles, they’re trying to reclaim the superpower status that Putin has always wanted to. It’s a great concern, it’s dangerous and it’s provocative,” Conaway told the Washington Times.

In addition to the U.S., the Russian strategic bombers have also intruded buffer zones around NATO countries, the Washington Times reported, adding that at least 24 such incidents occurred since 2012, in which a Russian bomber entered the U.S. or other NATO airspace.

Last month, the U.K. and Sweden reportedly scrambled their fighter jets to intercept Russian bombers near their borders. In late January, two Russian Bear long-range bombers that were seen flying near Britain's southern coast were intercepted by Royal Air Force (RAF) Typhoon fighter jets.

A similar incident occurred in February, when British RAF Typhoon fighters were deployed to intercept Russian Bear bombers that were spotted off the coast of Cornwall, U.K.

The latest report comes nearly a week after the Pentagon alleged that Russian warplanes had made several close passes in recent days over the U.S. destroyer USS Ross sailing in the Black Sea.

While the Russian media reportedly said that the USS Ross was acting “aggressively,” the U.S. Department of Defense argued that the vessel was “well within international waters at all times, performing routine operations.”

Meanwhile, Russia is reportedly working toward strengthening its air force. A senior official of the Russian air force recently said that the country would purchase at least 50 Tupolev Tu-160 “Blackjack” heavy strategic bombers after the aircraft's production is renewed.