The "routine" intercept of the four Russian planes -- which included Tu-95 bomber and Su-30 and Su-35 fighter aircraft -- took place on Tuesday, NORAD said in a statement.

"Russian aircraft remained in international airspace and did not enter American or Canadian sovereign airspace," it said.

This was the second intercept -- when an aircraft makes visual or electronic contact with another plane -- in two days, with the first taking place on Monday.

"NORAD... assesses that this Russian flight activity is in no way related to recent NORAD and US Northern Command operations associated with airborne objects over North America during the last two weeks," it added.

US aircraft shot down an alleged Chinese spy balloon and three unidentified objects this month -- rare offensive action for warplanes stationed in North America.