A U.S. naval officer says Russian military vessels and aircraft were spotted in the Black Sea near an area where NATO plans to hold a military exercise.
Rear Admiral Brad Williamson said the Russian Navy acted in line with international rules.
"They have their plans, we have ours," Williamson said, without disclosing further details.
He was speaking on March 8 to the Bulgarian BTA news agency aboard a U.S. cruiser docked in the Bulgarian Black Sea port of Varna.
The United States is due to take part in the exercise on March 9 alongside NATO partners Bulgaria, Romania, and Turkey amid tensions between the West and Russia over the Ukraine conflict.