The US warned that Russia is preparing to manufacture a reason to invade Ukraine, warning that an attack could happen in the coming days.
President Joe Biden said military action could begin imminently on Thursday, but stressed that a diplomatic solution was still possible.
His top foreign policy official later listed several ways Moscow could stage justification for an attack.
Russia said the claims were “baseless” and accused the US of stoking tensions.
It has repeatedly denied any plan to invade its eastern neighbour. Moscow insists it is moving troops away from the Ukrainian border, but that claim has been fiercely contested by Western powers.
Prime Minister Boris Johnson has claimed the shelling of a nursery school in the Donbas region of Ukraine by Russian-backed separatists was a “false-flag operation” aimed at discrediting the Ukrainian government.
According to the Organization for Security and Cooperation in Europe there were “multiple shelling incidents” on Thursday morning across the frontline in eastern Ukraine.
Three people were injured in the attack in the city of Stanytsia Luhanska, which blasted a hole through the wall of a nursery.
Volodymyr Zelenskiy, the Ukrainian president, accused the Russian side of “provocative shelling”.
But Mr Johnson said: “A kindergarten was shelled in what we are taking to be – well, we know – was a false-flag operation designed to discredit the Ukrainians, designed to create a pretext, a spurious provocation for Russian action.
“We fear very much that that is the kind of thing we will see more than over the next few days.”
Kindergarten worker Natalia Slesareva said her ears were ‘ringing’ after she was thrown against a door by a shell blast.
Children were sitting down to breakfast when a projectile blew a hole in the wall of a two-storey building being used by 20 youngsters and 18 staff.
Ms Slesareva said: “The children were eating breakfast when it hit.
“It hit the gym. After breakfast, the children had gym class. So another 15 minutes, and everything could have been much, much worse.”
The 58-year-old added: “The explosion wave blew me back against the door. Then there was smoke, dust, broken windows.”
“I could not feel the right side of my head. Everything was ringing.”
Mr Biden later told reporters outside the White House: “We have reason to believe they are engaged in a false flag operation to have an excuse to go in.”
A false flag is the term applied when a country stages an attack against its own interests in order to justify a retaliation. The US has for weeks been saying that such a misdirection is part of Russia’s plans. It was infamously deployed by Nazi Germany as a pretext for the invasion of Poland in 1939.
US Secretary of State Antony Blinken later told the United Nations Security Council that Russia was readying such a move.
He said it was not clear what form a pretext for an attack could take, but that possibilities include “a fabricated so-called terrorist bombing inside Russia, the invented discovery of a mass grave, a staged drone strike against civilians, or a fake – even a real – attack using chemical weapons”.
In the aftermath of such an incident, Mr Blinken said Russia’s government would likely “theatrically agree” an emergency meeting about protecting ethnic Russians in Ukraine – and Russian missiles and bombs would begin striking Ukrainian targets, alongside cyber-attacks.
The US has not provided evidence for the allegation, and Mr Blinken acknowledged that some people may question the claims. “But let me be clear. I am here today not to start a war but to prevent one,” he said.
The spike in tensions on Thursday came after a claims of an exchange of fire between Ukrainian forces and Russia-backed rebel extremists in the country’s east.
Ukrainian officials accused the separatists – who have been heavily armed by Russia – of targeting the nursery in Stanytsia Luhanska, firing shells including one which blasted through a wall into a children’s music room. Three adults but no children were injured.
Russia-backed rebels, meanwhile, said Ukrainian forces had shelled a number of locations under rebel control and accused them of escalating the conflict.
There are also competing claims about the build-up of troops along Ukraine’s border.
Russia has said that, following the completion of military exercises, some of its troops are now returning to their bases. It has published video purporting to show tanks boarding trains as evidence.
But US and other Western officials said thousands more troops are arriving and insist there is no evidence of any significant withdrawal. They say that claim is backed up not only by intelligence, but by widely available satellite photographs.
Satellite images revealed that Ukraine remains surrounded on three sides but are not seen as definitively prove that Russia is about to invade, which is vehemently denies.
On Thursday, it sent a formal response to US proposals for negotiation in the crisis which both offered a potential route to diplomacy and threatened further action.
In the document, Russia said it was open to discussions about inspections of missile sites on both sides but said the US had failed to address its main security concerns about the expansion of Nato near its border.
The possibility that Ukraine could one day join the Western military alliance is a key part of the ongoing crisis. Russia fiercely opposes such a move, but the US and Nato have not ruled it out.
Russia said that if that issue was not dealt with “military-technical means” it would be deployed in response – though it remains unclear what that could amount to.
And on the same day, it expelled the US deputy ambassador to Moscow. The US said the move was unprovoked and described it as a diplomatic escalation.
Despite such concerns, Ukraine’s Foreign Minister Dmytro Kuleba earlier said that “diplomacy is working.”
“We can never say for sure what happens tomorrow, but today we do our utmost” to maintain peace, he said. “We’re very much ready for any situation.”
A message from the Editor:
Thank you for reading this article. We’re more reliant on your support than ever as the shift in consumer habits brought about by coronavirus impacts our advertisers.
If you haven’t already, please consider supporting our trusted, fact-checked journalism by taking out a digital subscription.