Vladimir Putin has launched nuclear missile drills today as the US warns Russian troops on the Ukraine border are "poised to strike".
The exercises are being undertaken by Kremlin nuclear missile forces and come amid a series of military training sessions which the West fears are being used as a precursor for an invasion.
If Moscow sends troops into Ukraine there is major concern it could spark one of the worst conflicts since the Cold War.
US Defence Secretary Lloyd Austin said Russian forces were beginning to "uncoil and move closer" to the border with its former Soviet neighbour.
"We hope he steps back from the brink of conflict," he told a news conference on a visit to Lithuania, saying an invasion of Ukraine was not inevitable.
Russia ordered the military build-up while demanding NATO stop Ukraine ever joining the alliance but says predictions it is planning to invade Ukraine are wrong and dangerous.
It says it is now pulling back while Washington and allies insist the build-up is mounting.
Russian-backed separatist leaders in eastern Ukraine earlier declared a full military mobilisation, a day after ordering women and children to evacuate to Russia, citing the threat of an imminent attack by Ukrainian forces.
Kyiv flatly denied the accusation and both it and Washington say increased shelling across the ceasefire line this week is part of Russia's plan to create a pretext for an invasion of Ukraine.
Moscow said on Saturday a Ukrainian shell had exploded inside Russia and Russian news agencies later cited an unnamed source reporting a second.
Multiple explosions could be heard on Saturday morning in the north of the separatist-controlled city of Donetsk in eastern Ukraine, as more people got on buses to leave, a Reuters witness said.
The origin was not immediately clear. Ukraine said earlier that one of its soldiers had been killed.
"It's really scary. I've taken everything I could carry," said Tatyana, 30, who was boarding a bus with her four-year-old daughter.
U.S. President Joe Biden, who has given regular warnings of an impending invasion, said on Friday he now believes the capital Kyiv would be targeted by Russia but that he does not think Putin is even remotely contemplating using nuclear weapons.
Biden said Putin would invade in the coming days. "As of this moment, I am convinced that he has made the decision," he told reporters.
The Kremlin said Russia had successfully test-launched hypersonic and cruise missiles at sea and land-based targets during the exercises by Russia's nuclear forces.
Putin sat observing the exercises on screens along with the president of neighbouring Belarus, Alexander Lukashenko, from what the Kremlin described as a "situation centre".
Austin said the nuclear exercises were stoking concern among defence leaders around the world given that Russia's military was focused on a massive build-up of forces around Ukraine.
"When you layer on top of that a very sophisticated exercise with strategic nuclear forces, that makes things complicated to the degree that you could have an accident or a mistake," he said.
The drills follow a huge series of 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.
"The signal to the West is not so much 'don't interfere', but instead designed to say that the problem is not Ukraine and actually much wider," Dmitry Stefanovich, a research fellow at the IMEMO RAS think tank, said.
NATO Secretary-General Jens Stoltenberg said on Saturday Russia knew that 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 U.S.-based Maxar Technologies, which tracks developments with satellite imagery.
The Kremlin also has tens of thousands of troops staging exercises in Belarus, north of Ukraine, that are due to end on Sunday.
Lukashenko said on Friday they could stay as long as needed.
Ukrainian President Volodymyr Zelenskiy was set to meet U.S. Vice President Kamala Harris, Stoltenberg and other Western leaders at the annual Munich Security Conference on Saturday and return the same day amid fears Russia might try to engineer a coup.
Moscow has dismissed the idea it has any such plan.
Russian-backed rebels seized a swathe of eastern Ukraine in 2014, the same year that Moscow annexed the Crimea region from Ukraine after protests toppled its pro-Russian leader.
Kyiv says more than 14,000 people have since died in the conflict in the east.
In one breakaway region, Denis Pushilin, head of the self-proclaimed Donetsk People's Republic, said he had signed a decree on mobilisation and called on men "able to hold a weapon in their hands" to come to military commissariats.
The Luhansk People's Republic issued a similar decree.
Russian news agencies said on Saturday 10,000 evacuees had arrived so far in Russia. The separatist authorites say they aim to evacuate 700,000 people.
At a market in Donetsk, 38-year-old Oksana Feoktisova boarded an evacuation bus with her 9-year-old son and her mother.
They were accompanied by Feoktisova's brother Yuri who stayed behind in Donetsk.
"They don't let men on, and I wouldn't go anyway frankly," Yuri said. "I'm a reservist in any case. I'm an artillery man by birth... I'm loyal to my state, to my people."
