Claims by Russia's Ministry of Defence that its ships chased a US submarine out of its waters have been denied by Washington - sparking more fears the world is on the brink of war.
The Kremlin accused Washington of breaking international law, amid high levels of tension between the two superpowers over Vladimir Putin’s plans of invading Ukraine.
The Russian Ministry of Defence claimed in a release its Marshal Shaposhnikov frigate after detecting the sub near the Kuril Islands in the Pacific.
But US military spokesman Captain Kyle Raines said in a statement: "There is no truth to the Russian claims of our operations in their territorial waters.
"I will not comment on the precise location of our submarines but we do fly, sail, and operate safely in international waters."
Russia said the US vessel ignored orders to surface or leave the area, leading to the ship using “appropriate means” to drive it away.
The US embassy’s military attaché was summoned following the allegations.
The Russian fleet had been taking part in planned military drills at the time, including underwater control to prohibit anything entering its borders.
The Russian Defence Ministry said in a statement: "The U.S. submarine ... left Russian territorial waters at maximum speed."
A spokesman for the Pentagon said of the incident: “We are aware of press reporting about an alleged naval incident in the Pacific.
“We cannot confirm the details of these reports at this time.”
It comes as Russia has more than 130,000 troops massed near the Ukraine border and has sent troops to exercises in nearby Belarus.
Earlier Putin spoke to President Joe Biden on the phone, during which he was warned the US would “respond decisively and impose swift and severe costs on Russia” should they invade.
The White House said Mr Biden warned during the call an attack would "diminish Russia's standing" as the West pinned hopes on diplomacy to avert war.
They were said to have spoken for around an hour after French President Emmanuel Macron also shared a call with Mr Putin.
British, American and German citizens have been told to leave Ukraine immediately as fears grow lights out of the country may become difficult should an invasion begin.
US spies are said to have found plans about Putin’s invasion hopes which suggest it could start as early as next Wednesday.
Mr Biden's national security adviser Jake Sullivan said an attack before the end of the Winter Olympics on February 20 was a "credible prospect", assigning it a "very, very distinct possibility".
He warned a Russian attack on Ukraine could begin any day and would likely start with an air assault.
Mr Sullivan said new Russian forces were arriving at the border and are in a position to "mount a major military operation in Ukraine any day now", which could include a "rapid assault on the city of Kyiv" or on other parts of the country.
Speaking from the White House, he said Russia could choose "in very short order to commence a major military action against Ukraine".
Foreign Secretary Liz Truss said she discussed her "acute concerns" that Russia "may launch further military aggression against Ukraine in coming days" during a call on Saturday with US secretary of state Antony Blinken.
She said: "We agree Russia will face massive consequences for any invasion, including severe sanctions."
