U.S. fighter jets intercepted six Russian military planes in international airspace off the coast of Alaska on Monday, North American Aerospace Defense Command (NORAD) said Tuesday.

“Our ability to deter and defeat threats to our citizens and vital infrastructure starts with detecting, tracking, and positively identifying aircraft our airspace. We are on alert 24 hours a day, seven days a week, 365 days a year,” NORAD wrote in a tweet Tuesday attributed to its commander, Gen. Terrence O'Shaughnessy.

Our ability to deter and defeat threats to our citizens and vital infrastructure starts with detecting, tracking, and positively identifying aircraft our airspace. We are on alert 24 hours a day, seven days a week, 365 days a year - Gen. O'Shaughnessy, Commander NORAD — North American Aerospace Defense Command (@NORADCommand) May 21, 2019

On Monday, two Russian Tu-95 bombers entering the Alaskan Air Defense Identification Zone were intercepted by two U.S. F-22 fighter jets, NORAD said.

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A second group of two Russian Tu-95s and two Su-35 fighter jets were later intercepted by two more F-22s, NORAD said.

A U.S. E-3 spy plane provided overall surveillance, NORAD added.

Interceptions of Russian military planes in the Alaskan Air Defense Identification Zone are somewhat common. U.S. F-22s twice intercepted Russian bombers in September, following a similar interception in May 2018.

In its own statement Tuesday, the Russian Defense Ministry said its bombers were making “scheduled sorties” over neutral waters above the Chukchi, Bering and Okhotsk seas, as well as off the western coast of Alaska and the northern coast of the Aleutian Islands.

“At certain stages of the route, Russian aircraft were escorted by F-22 fighter jets of the USAF,” the statement said, referring to the U.S. Air Force.

The total flight time “exceeded 12 hours,” the ministry added.