But other senior U.S. defense and Navy officials stressed they did not see Russia's weekend bomber flights south of Japan as provocative.
Four U.S. fighter jets were scrambled on Feb. 9 to escort Russian bombers that approached the USS Nimitz south of Japan.
One Russian bomber flew over the deck of the aircraft carrier, escorted by a U.S. fighter jet.
Adm. Gary Roughead, U.S. chief of naval operations, downplayed the incident and said it reflected Russia's emerging naval power.
"I think what we are seeing is a Russian military or Russian navy that is emerging and, in the case of the navy, desiring to emerge as a global navy," Roughead told reporters at the Pentagon.
"I do not consider it to be provocative," he said of the bomber mission.
But on Capitol Hill, another top U.S. military officer -- Marine Corps Gen. James Cartwright -- said the Pentagon was trying to assess the implications of Russia's actions.
"Now, what we're concerned about is what are the indications of this return to a Cold War mind-set, what are the implications of that activity and how do we best address that," said Cartwright, vice chairman of the Joint Chiefs of Staff.
The incident happened in neutral international airspace, Cartwright said.
"We're just trying to go back and look at what message was intended by this overflight," he told a Senate panel.