Four Russian military aircraft were detected and tracked near US airspace on Thursday, the North American Aerospace Defense Command (NORAD) said.

The planes were spotted in the Alaska Air Defense Identification Zone (ADIZ), a buffer zone outside U.S. sovereign airspace. The US tracks incoming aircraft in the area for national security, and aircraft entering the zone must identify themselves.

“The Russian aircraft remained in international airspace and did not enter U.S. or Canadian sovereign airspace,” NORAD said in the statement.

NORAD noted that Russian aircraft activities in this area are regularly observed and are not considered a threat.

The organization said it tracks aircraft in the area through a “layered defense network of satellites, ground and airborne radars and fighter aircraft.”

It added that it “remains prepared to deploy a range of response capabilities in defense of North America.”

This is a breaking story. More to follow.