The Russian navy has started tracking a U.S. naval command vessel in the Black Sea, news agencies quoted the defense ministry as saying on Thursday, amid tension over NATO activities near Russia's borders.

"The forces and means of the Black Sea Fleet have started monitoring the actions of the Mount Whitney command ship, which entered the Black Sea zone on Nov. 4," TASS news agency quoted the defense ministry as saying.

The U.S. Navy said on Monday that the USS Mount Whitney had arrived in Istanbul and that it would soon join forces with other ships in the Black Sea.

President Vladimir Putin said on Monday that Russian forces could observe the Mount Whitney "through binoculars or in the crosshairs of (their) defense systems."

On Tuesday, the Black Sea fleet said its ships had rehearsed destroying enemy targets and that air defense systems had been put on alert at its bases in Novorossiysk and on Crimea, the Black Sea peninsula which Russia annexed from Ukraine in 2014.

Russia, which severed ties with NATO last month, complains frequently about alliance activity close to its borders or in what it regards as its post-Soviet sphere of influence.