Russian military flights over European airspace are raising tensions and pose a potential danger to security and to civilian aircraft, a US military spokesman said Friday.

The United States was concerned about the flights of Russian warplanes and was tracking the activity "very closely," Rear Admiral John Kirby told reporters.

"We certainly don't see these increased flights and activity as helpful to the security situation in Europe. Clearly, they pose the potential risk of escalation," he said.

The flights also represented "a potential risk to civil aviation just in the sheer number of and size of and scope of these flights," Kirby said.

There was another round of flights on Friday, with NATO monitoring Russian planes over the Baltics, the North Sea and the Atlantic Ocean.

"The aircraft involved fighter jets, long-range bombers and tanker aircraft," Kirby said.

He added that the United States wants Russia "to take steps, concrete tangible steps to reduce tension, not increase it."

NATO this week said Russian military aircraft engaged in large-scale flight operations in European airspace and alliance planes were sent up to intercept and identify them.

NATO chief Jens Stoltenberg said Thursday that the transatlantic alliance remained vigilant in the face of the Russian flights.

The air activity follows Russia's military intervention in Ukraine, which NATO has condemned, calling it the most serious threat to transatlantic security since the Cold War.

NATO says there have been more than 100 intercepts of Russian aircraft in 2014 to date, about three times the number in 2013.