The Russian Air Force's nuclear-capable bombers reportedly conducted at least 16 incursions into and around north-western US air defence identification zones in the last ten days.
North American Aerospace Defense Command and the US Northern Command spokeswoman major Beth Smith was reported by The Washington Free Beacon as saying that the Russian aircraft involved in incursions included a mix of Tu-95 Bear H heavy bombers, Tu-142 Bear F maritime reconnaissance aircraft, as well as an IL-20 intelligence collection aircraft.
The incursions primarily took place along the Alaskan air defence identification zone, while one of the flights was also found to be entering into Canada's air defence zone, Smith added.
The spokeswoman called the flights a 'spike in activity', and noted that they were assessed as routine training missions and exercises.
However, a US defence official said the Russian flights did not appear to be 'just training missions'.
The Russian strategic nuclear forces appear to be 'trying to test our air defence reactions, or our command and control systems', the official added.
The flights incursion by Tu-95 Russian Bear H bombers prompted the scrambling of US jet fighters on several occasions, according to the news agency.
Meanwhile, undisclosed US defence officials said the increased Russian aerial incursions are very unusual and goes back to the Cold War era.
The US military has earlier confirmed that its F-22 and F-15 fighters intercepted four long-range Russian Tu-95 Bear H bombers flying close to Alaska, Northern California coast on 9 May.
In addition, a Russian armed SU-27 Flanker fighter was reported to have flown dangerously close to a US Air Force's RC-135U intelligence-gathering aircraft over international waters between Russia and Japan, in April.