These are the spy planes Putin keeps sending past Alaska

Russian military aircraft — everything from long-range bombers to advanced fighters to spy planes — have ventured close to Alaska three times in one month, twice prompting US F-22 stealth fighters to intercept the aircraft.

Two pairs of unidentified Russian maritime reconnaissance aircraft flew past northern Alaska Sept. 21, 2018, the North American Aerospace Defense Command (NORAD) announced in a statement, noting that while the surveillance aircraft entered the Alaskan Air Defense Identification Zone, they remained in international airspace.

The flight follows an alarming incident Sept. 20, 2018, in which two unresponsive Russian Tu-160 bombers approached the British coastline, causing France and the UK to scramble fighters to intercept the supersonic aircraft.

"Russian bombers probing UK airspace is another reminder of the very serious military challenge that Russia poses us today," Defence Secretary Gavin Williamson said in a statement, adding, "We will not hesitate to continually defend our skies from acts of aggression."

A Russian Tupolev Tu-160.

Japan encountered similar problems on Sept. 19, 2018, when it sent fighters to intercept Russian fighter jets approaching Japanese airspace. The same thing happened in early September 2018.

Two Russian Tu-95 Bear bombers accompanied by Su-35 Flanker fighter jets approached western Alaska on Sept. 11, 2018, leading the US to dispatch two F-22s in response.

A similar incident occurred on Sept. 1, 2018, when two of the same type of bomber entered the Alaskan Air Defense Identification Zone south of the Aleutian Islands. Alaska-based NORAD F-22 fighters were sent out to deal with that situation as well.

Following the first incident early September 2018, American defense officials speculated that the Russian bombers may have been practicing for possible cruise missile strikes on US missile defense systems in Alaska, although the true purpose of the flights is difficult to discern.

While seemingly disconcerting, Russia does this sort of thing fairly regularly. Russian Tu-160 bombers flew past Alaska in August 2018, and another pair of bombers did the same in May 2018. These flights come at a time in which tensions between Moscow and Washington are on the rise.

This article originally appeared on Business Insider. Follow @BusinessInsider on Twitter.