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Four Russian bombers entered the U.S. Air Defense Zone near Alaska on Monday, senior defense officials told NBC News on Thursday.

U.S. fighter jets from Alaska responded to shadow the Russian craft, which were Tupolev-95 "Bear" four-engine turboprop bombers, the officials said.

The four bombers split into two separate groups of twos: One group remained in the vicinity of Alaska, the other headed south off the northern coast of California, the officials said.

The Air Defense Zone is the area around the United States and Canada wherein aircraft are required to identify themselves.

A Russian Tupolev Tu-95 bomber flies over international waters near the coast of Norway in 2007.Scanpix Norway / REUTERS file

The officials stressed that at no time did the bombers violate actual U.S. airspace and added that for the U.S. military, these forays are considered rather routine.

There have been about 50 such intercepts of Russian bombers over the past five years.

— Jim Miklaszewski and Courtney Kube