Russian president Vladimir Putin launched exercises by Russia's strategic nuclear missile forces on Saturday and Washington said Russian troops massed near Ukraine's border were moving forward and "poised to strike".
The Kremlin said Russia had successfully test-launched hypersonic and cruise missiles at sea at land-based targets during its military exercises.
Mr Putin observed the exercises on screens with Belarusian leader Alexander Lukashenko from what the Kremlin called a "situation centre".
The drills involved launches from warships, submarines and warplanes as well as from land that struck targets on land and at sea, the Kremlin said.
Two ballistic missiles were launched – one from a site in northwest Russia and the second from a submarine in the Barents Sea – hitting targets thousands of miles away in the far east peninsula of Kamchatka, it said.
Russia’s RIA news agency aired footage showing a split screen of various top senior military chiefs as well as Mr Putin, who ordered the drills to begin.
The Kremlin has said the exercises are part of a regular training process and denied they signal an escalation of the standoff.
In the footage aired by RIA, Chief of the General Staff Valery Gerasimov told Mr Putin: "The main purpose of the exercise is to train the strategic offensive forces' actions aimed at delivering a guaranteed defeat of the enemy."
Ships and submarines from the Northern and Black Sea Fleets launched Kalibr cruise missiles and Zircon hypersonic missiles at sea and land targets, the Kremlin said.
The Russian defence ministry released footage of aircraft launching a Kinzhal hypersonic ballistic missile and striking a land target.
US Defence Secretary Lloyd Austin said the nuclear exercises were stoking concern among defence leaders around the world as Russia's military was focused on a huge build-up of forces around Ukraine and there was a danger of "an accident or a mistake".
The drills follow manoeuvres by Russia’s armed forces in the past four months that have included a build-up of troops – estimated by the West to number 150,000 or more – to the north, east and south of Ukraine.
Moscow-based analysts said the exercises were aimed at sending a message to take Russia's demands for security guarantees from Nato seriously after the alliance's expansion to Russia's borders since the collapse of the Soviet Union in 1991.
"Ignoring Russia's legitimate rights in this area adversely affects the stability not only on the European continent, but also in the world," Russian foreign minister Sergei Lavrov was quoted by his ministry as telling France's foreign minister by phone.
Nato Secretary-General Jens Stoltenberg said Russia knew the alliance could not meet its demands, which include the withdrawal of Nato forces from former communist east European states that have elected to join Nato.
New helicopters and a battle group deployment of tanks, armoured personnel carriers and support equipment have deployed in Russia, near the border, according to US-based Maxar Technologies, which tracks developments with satellite imagery.
The Kremlin also has troops staging exercises in Belarus, north of Ukraine, that are due to end on Sunday. Mr Lukashenko said on Friday they could stay as long as needed.
Ukrainian president Volodymyr Zelenskiy met US vice-president Kamala Harris, Stoltenberg and other Western leaders at the annual Munich Security Conference on Saturday. He said Ukraine was looking for peace. – Reuters