“We certainly don’t see these increased flights and activity as helpful to the security situation in Europe,” Kirby told reporters on Friday.
President Putin should “take concrete, tangible steps to reduce tension, not increase it,” he added.
The warning was issued after NATO on Wednesday announced that an increasingly large number of Russian military aircraft have been tracked flying unannounced into European airspace this month.
The alliance claimed at least four groups of Russian bombers conducted “significant military maneuvers in European airspace” over the Baltic, Black Seas, and the Atlantic Ocean this week.
NATO said in a press release the flights represented an "unusual level of air activity over European airspace.”
Kirby said that the flights pose a “potential risk of escalation” between nations because of the “sheer number of and size of and scope of these flights.”
The Pentagon’s chief spokesman also noted that his country was “very, very closely” watching the sorties.
The incident comes amid heightened US-Russia tensions over the current crisis in Ukraine.
The United States accused Russia of supporting pro-Russian forces in Ukraine, an allegation denied by the Kremlin.