The North American Aerospace Defense Command said in a statement Wednesday that the first formation of Russian aircraft consisted of two bombers, accompanied by two fighter jets, and was supported by an airborne early warning and control aircraft.

The second formation consisted of two Russian bombers supported by another airborne early warning and control aircraft. The Russian military aircraft came within 32 nautical miles of Alaskan shores, but remained in international airspace and at no time did they enter US sovereign airspace.

"These are significant Russian formations in multiple locations," a US defense official said of the two recent intercepts. The US assesses that some of the Russian flights are simply training missions. But the pace of activity may be a signal by Moscow that they are determined to keep pace with American flights off Russian coastal regions, the official said.

CNN's Devan Cole and Chandelis Duster contributed to this report.