Russia rehearsed a nuclear strike on Wednesday with the world on high alert for its next move in Ukraine.
President Vladimir Putin monitored the drills, which simulated a response to a nuclear attack on Russia.
Mr Putin “conducted an exercise for ground, marine and air strategic defence forces, which included launches of ballistic and cruise missiles,” the Kremlin said in a statement.
US officials said said they were notified of the drills in advance, amid repeated threats of nuclear force from Moscow during the war in Ukraine.
Ukraine meanwhile said its counter-offensive in the southern Kherson region was being bogged down by bad weather.
"We have a lot of irrigation and water supply channels, and the Russians use them like trenches," Defence Minister Oleksii Reznikov said.
In Russia, state television showed Mr Putin overseeing the nuclear drills from a control room.
They involved the test-firing of a Yars land-based intercontinental ballistic missile from Plesetsk, Russia's northern launch site, and the launch of a Sineva intercontinental ballistic missile by a Russian nuclear submarine in the Barents Sea.
Air-launched cruise missiles were also deployed with Tu-95 strategic bombers launching their ammunition at practice targets, Moscow said.
All missiles reached their target, it added.
Defence Minister Sergei Shoigu reported to Mr Putin that the drills were simulating a “massive nuclear strike” by Russia in retaliation for a nuclear attack.
Russia has been warned of “severe consequences” if it orders the use of nuclear weapons in Ukraine, although western powers are keeping quiet on what these might be.
US President Joe Biden warned Moscow on Tuesday that a nuclear escalation would be an “incredibly serious mistake”.
However, the Pentagon and State Department said Russia had complied with its arms control obligations by giving warning of the missile launches.
"While Russia engages in unprovoked aggression and reckless nuclear rhetoric, these notification measures do ensure we are not taken by surprise," said State Department spokesman Ned Price.
Russian military chiefs claim Ukraine is developing such a device and could use it to frame Russia for a nuclear attack.
"Allies reject this blatantly false accusation, and Russia must not use false pretexts to escalate the war further," Mr Stoltenberg told reporters at Nato headquarters in Brussels.
In a meeting with senior French and British officials, Ukraine called the Russian allegation "completely groundless" and said it was part of a Kremlin disinformation campaign.
Russia announced the annexation of four regions of Ukraine last month and hinted that it could use nuclear force to defend what it now considers its territory.
Western powers rejected the annexations as illegal and invalid and the referendums that led to them as a charade.