The United Nations has urged restraint by all parties in eastern Ukraine amid fresh shelling that violated a ceasefire under the Minsk Agreement.
"If verified, these must not be allowed to escalate further. We call on all sides to exercise maximum restraint at this sensitive time," Rosemary DiCarlo, the UN undersecretary general, told a Security Council meeting on Ukraine.
US President Joe Biden has said there was now "every indication" that Russia was planning to go into Ukraine, including signs Moscow was preparing a "false flag" operation to justify it.
Ukraine and pro-Russian separatists within Ukraine earlier exchanged fire across a frontline that divides them, in what Western officials described as a possible pretext created by Moscow to invade.
Mr Biden ordered Secretary of State Antony Blinken to change his travel plans at the last minute to speak at a United Nations Security Council meeting on Ukraine.
"The evidence on the ground is that Russia is moving toward an imminent invasion. This is a crucial moment," US Ambassador to the United Nations, Linda Thomas-Greenfield, told reporters.
Russia denies planning to invade its neighbour and said this week it was pulling back some of the more than 100,000 troops it has massed near the frontier. The US said Russia is not withdrawing, but in fact sending more forces.
"We see them fly in more combat and support aircraft. We see them sharpen their readiness in the Black Sea," Defense Secretary Lloyd Austin said at NATO headquarters in Brussels.
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"We even see them stocking up their blood supplies.
"I was a soldier myself not that long ago. I know firsthand that you don't do these sorts of things for no reason," said Mr Austin, a retired army general.
"And you certainly don't do them if you're getting ready to pack up and go home."
Ukraine and pro-Russian rebels gave conflicting accounts of shelling across the front in the Donbass separatist region. The details could not be established independently, but reports from both sides suggested an incident more serious than the routine ceasefire violations reported regularly in the area.
Kremlin spokesperson Dmitry Peskov said Russia was "seriously concerned" about reports of an escalation.
Britain's Foreign Secretary Liz Truss called the reports "a blatant attempt" by the Russian government to fabricate pretexts for invasion.
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Ukraine's President Volodymyr Zelensky said the pro-Russian forces had shelled a playschool, in what he called a "big provocation".
The separatists, for their part, accused government forces of opening fire on their territory four times in the past 24 hours.
Mr Austin said Washington was "still gathering details, but we have said for some time that the Russians might do something like this in order to justify a military conflict".
NATO Secretary General Jens Stoltenberg said the alliance was "concerned that Russia is trying to stage a pretext for an armed attack against Ukraine. There is still no clarity, no certainty about the Russian intentions".
"They have enough troops, enough capabilities to launch a full-fledged invasion of Ukraine with very little or no warning time."
A senior Ukrainian government source said the shelling at the line of contact with Russian-backed separatist forces went beyond the scale of ceasefire violations routinely reported throughout the conflict.
'Looks a lot like provocation'
"It is not typical. It looks a lot like a provocation," the source told Reuters.
Mr Peskov said Russia had already warned that a concentration of additional Ukrainian forces near the Donbass frontline created a risk of provocations. Kyiv has denied massing extra troops in the area.
Kyiv accused the rebels of firing shells at several locations, including some that struck a kindergarten and others that hit a school where pupils had to flee to the cellar.
Video footage released by Ukrainian police showed a hole through a brick wall in a room scattered with debris and children's toys. Separate images showed emergency workers escorting small children and teachers from a building.
Russia's defence ministry released video it said showed more Russian units leaving the area near the border.
But Maxar Technologies, a private US company that has been tracking the build-up, said satellite images showed that, while Russia has pulled back some military equipment from near Ukraine, other hardware has arrived.
'No justification for massing of troops' - Martin
The Taoiseach has said there is no justification for "the massing of so many Russian troops" on the border of Ukraine and that civilians need respite from this threat of conflict.
Micheál Martin was speaking in Brussels where leaders of EU and African Union countries have gathered for a two day summit.
He said he expected briefings today on the situation but did not anticipate there will be any decisions made today on sanctions.
Taoiseach Micheál Martin has said the EU has made it clear that if Russia invaded Ukraine, there would be a united response in the form of sanctions.
— RTÉ News (@rtenews) February 17, 2022
He said: "If Russia was to invade Ukraine, sanctions would follow and follow very quickly" | More: https://t.co/rhissuHVU3 pic.twitter.com/ohQrtUczge
However he said the EU has made it clear that if Russia invades Ukraine there will be a united response that will be robust and in the form of sanctions.
"If Russia was to invade Ukraine, sanctions would follow and follow very quickly," he said.
He acknowledged sanctions would have an impact on everyone including Ireland.
He said they are calling for de-escalation and dialogue and diplomacy is important.
"Our hearts go out to the people of Ukraine who are under a lot of pressure right now from what is a very intimidatory and threatening massing of significant troops on their border."
He said from his perspective there is no threat to Russia "right now from the Ukraine or anybody else".
Mr Martin said EU leaders want to de-escalate this situation and they certainly do not want conflict.
"Personally, I have no understanding of why we get to situations like this and there is no need for this."
He said there are various processes through which these issues can be resolved through diplomacy and dialogue.
Additional reporting Tony Connelly
