Russia announced massive drills of its nuclear forces on Friday as tensions with the West mount over allegations it has some 150,000 troops on Ukraine's border.
The Russian defence ministry said that President Vladimir Putin would supervise exercises of the strategic forces with the firing of ballistic and cruise missiles.
The drills aim to "test the state of readiness" of the forces involved and the "reliability of strategic nuclear and non-nuclear weapons", the ministry said.
US President Joe Biden had given a dire warning a day previously, stating that analysts did not see signs of de-escalation.
"Every indication we have is they’re prepared to go into Ukraine, attack Ukraine," Biden told reporters at the White House.
He said the US has “reason to believe” that Russia is “engaged in a false flag operation to have an excuse to go in," without providing more detail.
Jens Stoltenberg, secretary-general of NATO, told Euronews that Russia could stage a "false-flag operation" in order to justify an incursion over the border.
"We have seen attempts by Russia to [...] stage an event and then use that as an excuse for launching an attack on Ukraine. And of course, that adds to the seriousness of the situation," he said.
The Kremlin has repeatedly said that it has no plans to invade Ukraine but has demanded assurances from the West that NATO does not expand to Ukraine or other former Soviet Republics.
US defence secretary Lloyd Austin, who is in Europe to discuss the crisis, questioned recent Russian claims that some troops would withdraw from the border.
“We’ve seen some of those troops inch closer to that border,” he said. “We even see them stocking up their blood supplies. You don’t do these sort of things for no reason, and you certainly don’t do them if you’re getting ready to pack up and go home.”
Some are concerned that a possible pretext for an invasion could be related to the separatist conflict in the east that saw shelling and apparent cyberattacks in the past two days.
Monitors from the Organisation for Security and Cooperation in Europe reported more than 500 explosions before the tensions eased in the evening, with both Ukrainian authorities and separatists trading accusations of over the shelling.
NATO defence ministers met on Thursday and discussed strengthening their defences in Eastern Europe amid fears that the violence in eastern Ukraine could spark a wider conflict.