A Russian warship has caught a US Navy destroyer operating in Russia's Pacific waters and chased it off, the defence ministry in Moscow said.
The USS John S McCain, a guided-missile destroyer, violated Russia's territorial waters by moving two kilometres beyond the maritime border at 0317 GMT yesterday into Peter the Great Gulf, the ministry said in a statement.
The US ship had been operating for several days in the Sea of Japan and was being tracked by Russia, the ministry said.
The Admiral Vinogradov, an anti-submarine destroyer in Russia's Pacific Fleet, sent a warning to the US ship and threatened to "use a ramming manouevre to drive the intruder out of its territorial waters".
The US vessel immediately returned to neutral waters, the statement said, and was being monitored.
The US Pacific Fleet said in a statement that the USS John S McCain had "asserted navigational rights" in the area "by challenging Russia's excessive maritime claims".
It gave no details of the incident but said the US ship had been operating in the vicinity of the Peter the Great Gulf in an area the United States does not recognise as Russian territorial waters.
The area has been in dispute since 1984, when the Soviet Union declared it part of its waters, a claim that Russia has maintained.
Such incidents at sea are uncommon and a sign of continued post-Cold War tensions between Russia and the United States.
Last year the two countries traded accusations of dangerous manoeuvres after the Admiral Vinogradov came close to a collision with the USS Chancellorsville in the East China Sea.