MOSCOW/KYIV- U.S. President Joe Biden said on Thursday 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 on Thursday across a frontline that divides them, in what Western officials described as a possible pretext created by Moscow to invade.

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," U.S. 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. Washington says 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. "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 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 Moscow 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.

Ukraine's President Volodymyr Zelenskiy said the pro-Russian forces had shelled a kindergarten, 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.

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 A PROVOCATION'

"It is not typical. It looks a lot like a provocation," the source told Reuters.

A Reuters photographer in the town of Kadiivka, in Ukraine's rebel-held Luhansk region, heard the sound of some artillery fire from the direction of the line of contact, but was not able to determine the details of the incident.

Kremlin spokesperson Peskov said Moscow 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 U.S. 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. 

(Reporting by Reuters bureaux; Writing by Peter Graff and Mark Trevelyan; Editing by Alex Richardson and Philippa Fletcher)