Four Russian military aircraft entered the Alaska Air Defense Identification Zone (ADIZ) on Monday, according to the North American Aerospace Defense Command (NORAD).

The aircraft remained in international airspace and did not breach American or Canadian sovereign airspace, NORAD confirmed in a press release, adding that no intercept was required.

“This Russian activity in the Alaska ADIZ occurs regularly and is not viewed as a threat,” the statement said.

Monday’s detection marks the fifth such incident in September. While NORAD did not specify the type of Russian aircraft involved this time, earlier this month, Russian IL-38 maritime patrol aircraft were identified inside the Alaska ADIZ.

The ADIZ begins at the edge of national airspace and requires all aircraft to be identified for national security purposes, according to NORAD. The command uses a layered defense system of satellites, ground-based and airborne radars, and fighter jets to track and respond to potential threats.

“NORAD remains ready to employ a number of response options in defense of North America,” the statement concluded.