The North American Aerospace Defense Command (NORAD) reported the detection of Russian military aircraft off the coast of Alaska on Sunday, marking the fourth such instance since the September 11 attacks amid heightened tensions between the United States and Russia.

Two Russian IL-38 military planes operating within the Alaska Air Defense Identification Zone (ADIZ) were detected and tracked on Sunday, NORAD stated in a press release.

The aircraft remained in international airspace and did not enter U.S. airspace, NORAD confirmed, adding that this Russian activity in the Alaska ADIZ occurs regularly and is not considered a threat.

The aircraft involved in the other three incidents in the past week — on Wednesday, Friday, and Saturday — also remained in international airspace and were not deemed a threat, according to NORAD.

The U.S. and Canada, jointly responsible for NORAD, first intercepted a pair of Russian military aircraft on Wednesday.

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

An ADIZ extends beyond sovereign airspace and encompasses a defined area of international airspace requiring the identification of all aircraft for national security purposes, NORAD explained.

NORAD employs a layered defense network of satellites, ground-based and airborne radars, and fighter aircraft to detect and track aircraft, guiding appropriate actions. The organization stated its preparedness to utilize various response options to defend North America.

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.