The North American Aerospace Defense Command, or NORAD, said it detected Russian military aircraft off the coast of Alaska on Sunday, marking the fourth time since 9/11 amid heightened tensions between the U.S. and Russia.

Two Russian IL-38 military planes operating in the Alaska Air Defense Identification Zone, or ADIZ, were detected and tracked on Sunday, NORAD said in a news release.

The aircraft remained in international airspace and did not enter U.S. or Canadian sovereign airspace, NORAD said, adding that this Russian activity in the Alaska ADIZ happens regularly and is not believed to be a threat.

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Two Russian military aircraft operating in the Alaska Air Defense Identification Zone were detected and tracked on Sunday, the fourth time in the past week. (Getty Images)

The aircraft in the other three incidents in the past week — on Wednesday, Friday and Saturday — also remained in international airspace in Alaska's ADIZ and were not viewed as a threat, according to NORAD.

The U.S. and Canada, which together make up NORAD, first intercepted a couple of Russian military aircraft on Wednesday.

A pair of TU-142s were detected by NORAD on Friday. And on Saturday, two Russian IL-38 planes were intercepted, the same type of aircraft intercepted in Sunday's incident.

The aircraft in all four incidents remained in international airspace and did not enter U.S. or Canadian sovereign airspace. (Getty Images)

An ADIZ begins where sovereign airspace ends and is a defined stretch of international airspace requiring the ready identification of all aircraft in the interest of national security, NORAD said.

NORAD said it employs a layered defense network of satellites, ground-based and airborne radars and fighter aircraft to detect and track aircraft and inform appropriate actions. The company said it is prepared to employ a number of response options to defend North America.

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The aircraft in all four incidents were not seen as a threat, NORAD said. (Getty Images)

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Earlier this summer, NORAD intercepted two Russian and two Chinese bombers flying near Alaska for the first time that the two countries have been intercepted while operating together, U.S. officials said at the time.