American fighter jets intercepted four Russian military aircraft flying in international airspace west of Alaska this week, officials said.

Two Russian Tu-95 "Bear" bombers and two Su-35 "Flanker" fighter jets entered what is known as the Alaska Air Defense Identification Zone on Tuesday, NORAD said in a statement Wednesday.

NORAD did not say how close to American airspace the Russian aircraft flew nor in which direction. It said the intercept happened Tuesday at 10 p.m. ET. Russian state media said the American jets escorted its aircraft for around 40 minutes and never came closer than 100 meters, according to Reuters.

This air defense zone — known as an ADIZ — was declared unilaterally by the U.S. after World War II and is not binding under any international treaty. It extends more than 200 miles farther from Alaska's westernmost island and 1,700 miles from Anchorage.

It was not the first time such an incident has happened. This is the second similar interception this month after American fighter jets briefly escorted Russian bombers west of Alaska on Sept. 1.

In recent years, Russia has been accused of flying dangerously close to U.S. aircraft and ships in eastern Europe's Black Sea, and repeatedly violating the airspace of U.S. NATO allies in the Baltics.

"Homeland defense is NORAD's top priority," NORAD's statement said after Tuesday's interception. "The identiﬁcation and monitoring of aircraft entering a U.S. or Canadian Air Defense identiﬁcation Zone demonstrates how NORAD executes its aerospace warning and aerospace control missions for the United States and Canada."

Gen. Terrence J. O'Shaughnessy, the NORAD commander, added, "The homeland is no longer a sanctuary, and the ability to deter and defeat threats to our citizens, vital infrastructure, and national institutions starts with successfully detecting, tracking, and positively identifying aircraft of interest approaching US. and Canadian airspace."

The U.S. was the first to declare an ADIZ in the 1950s and around 20 other countries have since followed suit.

Under its rules, the U.S. requests any foreign aircraft to identify itself and its flight plan when entering the air defense zone.

The FAA warns foreign aircraft doing so that they may be intercepted by fighter jets, during which time they are asked to establish radio contact and follow instructions.

"Be advised that noncompliance may result in the use of force," the FAA says.