Russian Tu-99 bombers fly in formation during rehearsals for the Victory Day military parade, with a tower of the Kremlin in the foreground, in Moscow, May 3, 2014.
Russian long-range bombers crossed into the U.S. air defense identification zone earlier this week, but exited before entering U.S. air space.
Pentagon spokesman Colonel Steve Warren said one Russian refueling tanker and four bombers were visually identified by U.S. fighter planes Monday near Alaska.
Two bombers exited the identification zone, while the other two bombers headed south near the northern California coast.
The air defense identification zone extends beyond the United States' airspace to give the country more time to respond to foreign aircraft. Warren says the military has seen about 50 of these crossings during the past five years.