Russian military officials say a US submarine has been driven away after being found loitering near naval exercises.
Aleksandr Peletsky, a spokesman for the Russia's north-eastern group of forces, said the Los Angeles class submarine was in Russian territorial waters and was believed to have been spying.
However, naval command in Moscow told AFP news agency that the submarine was within Russia's 200-kilometre economic zone but did not violate its territory.
Ships and aircraft from the Russian Pacific Fleet were scrambled to escort the submarine out to sea from the Kamchatka peninsula's Avacha Bay.
It is not clear whether any further action will be taken over the incident.
More than 10,000 troops, 25 ships and submarines, and 20 aircraft have been taking part in the manoeuvres.
Air defence, electronic warfare and chemical defence units were also involved.
During the Cold War, American submarines regularly entered Soviet waters around Kamchatka and the Sea of Okhotsk.
Washington relied heavily on submarines for espionage, using them to monitor Soviet harbours, observe missile tests and even retrieve top secret debris from the bottom of the sea.
US documentation of such activities remains classified, but servicemen involved have spoken of dangerous cat-and-mouse games which sometimes led to collisions.