Washington (CNN) -- Russian bomber aircraft were spotted last week off Alaska and then within 50 miles of California, military officials said.
Four long-range planes flew close to Alaska on June 4 where they were intercepted by U.S. F-22 fighter jets. Two peeled off and headed west, while the other two flew south and were identified by American F-15 fighters within 50 miles of the California coast.
Details were confirmed to CNN on Thursday by the Defense Department and the North American Aerospace Defense Command, a joint U.S.-Canadian force.
The United States believes the Russians were on a training mission, Pentagon spokesman Col. Steve Warren said.
The planes, which included Tu-95 Bear-H bombers and refueling aircraft, never breached U.S. air space, Capt. Jeff Davis, a NORAD spokesman, said.
But they did enter the U.S. Air Defense Identification Zone, which is an area extending 200 miles from the North American coast and is mainly within international airspace. Commercial and other planes must identify themselves when they fly through that zone.
"They followed all the protocols and it was a very professional encounter on both sides," Davis said. "There was nothing that they were doing that was contrary to international law."
NORAD intercepts Russian bombers in the defense zone about 10 times a year, Davis said, though usually not near California.
Russian military planes were last intercepted off the coast of California on July 4, 2012, Davis added.
CNN's Barbara Starr, Jeremy Diamond, and Joshua Berlinger contributed to this report.