NORAD (North American Aerospace Defense Command) said that it detected Russian military planes off the coasts of Alaska Sunday. This is the fourth detection since 9/11, amid increased tensions between Russia and the United States.

NORAD announced in a press release that two Russian IL-38 aircraft were tracked and detected on Sunday in the Alaska Air Defense Identification Zone (ADIZ).

NORAD reported that the aircraft did not cross into U.S. airspace or Canadian airspace. It also noted that Russian activities in Alaska ADIZ occur regularly and are not considered a threat.

NORAD reports that the aircraft involved in the three other incidents of the past week – on Wednesday, Friday, and Saturday – also remained within international airspace and the ADIZ in Alaska and were not considered a threat.

On Wednesday, the U.S., Canada, and NORAD together intercepted two Russian military aircraft.

NORAD detected two TU-142s on Friday. On Saturday, NORAD intercepted two Russian IL-38 aircraft, the same type as those intercepted on Sunday.

NORAD stated that an ADIZ is defined as the area where the sovereign airspace ends. It is a stretch of airspace which requires the identification of aircraft for national security.

NORAD has said that it uses a multi-layered defense system of satellites, airborne and ground radars, and fighter aircraft to track and detect aircraft and take appropriate action. The company stated that it is ready to use a variety of response options to defend North America.

NORAD intercepted, earlier this summer, two Russian and two Chinese aircraft flying near Alaska. This was the first time the two countries had been intercepted together.