The Russian Navy is tracking the first U.S. Navy warships to have entered the Barents Sea near Norway in more than 30 years, the Defense Ministry said Monday.

The guided-missile destroyers USS Donald Cook and USS Porter, the large supply vessel USNS Supply and the British frigate HMS Kent entered the Barents Sea after conducting anti-submarine drills in the Norwegian Sea. The destroyer USS Roosevelt joined them in the Barents Sea for the security operation that the U.S. Navy called an effort “to assert freedom of navigation and demonstrate seamless integration among allies.”

U.S. Navy surface ships have not operated in the Barents since the mid-1980s,” the U.S. 6th Fleet said in a statement Monday. 

“In these challenging times, it is more important than ever that we maintain our steady drumbeat of operations across the European theater, while taking prudent measures to protect the health of our force,” said its commander, Vice Adm. Lisa Franchetti.

The fleet said it had notified Russia’s Defense Ministry of the visit last Friday “in an effort to avoid misperceptions, reduce risk and prevent inadvertent escalation.”

“The Northern Fleet’s assets have begun tracking NATO’s surface action group,” Interfax quoted the Defense Ministry’s national defense management center as saying.

The defense management center said the NATO warships entered the Barents Sea at 7 a.m. Moscow time.

Russia and NATO member Norway have both stepped up their military presence in the area after Russia annexed Crimea in 2014.

Norway’s absence from the 6th Fleet’s operation in the Barents Sea is an attempt to avoid escalating tensions with Russia, The Barents Observer reported Monday.

Norway abstained from these exercises perhaps understanding the danger to its bilateral relationship with Russia from threatening Russia’s nuclear deterrence,” it quoted Rasmus Gjedssø Bertelsen, social sciences professor at the University of Tromsø – The Arctic University of Norway, as saying.