Ukraine and Moscow-backed separatists traded accusations Thursday of intensifying shell fire across their frontline, fuelling fears of a potential Russian invasion of its pro-Western neighbour.
Ukraine has been in conflict with rebels in the eastern Donetsk and Lugansk regions since 2014, in a war that has cost thousands of lives.
But the new reports of intense shelling came as fears grow that the situation could escalate into a wider conflict.
Tensions between Russia and the West have reached a post-Cold War high over claims Moscow is planning an invasion of Ukraine, despite Russia saying again Thursday that it was pulling back more troops from near the border.
The Ukrainian army accused Russian-backed separatists of 34 ceasefire breaches on Thursday, 28 of them using heavy weapons.
It said that two Ukrainian soldiers and five civilians had been injured, including three adults wounded by artillery fire that hit a kindergarten in the village of Stanytsia-Luganska while children were inside.
"The shelling of a kindergarten... by pro-Russian forces is a big provocation," President Volodymyr Zelensky said on Twitter, calling for international observers to remain in Ukraine to monitor a fragile ceasefire.
Russian news agencies meanwhile quoted authorities in the separatist Lugansk region saying they blamed Kyiv after the situation on the frontline "escalated significantly".
In Moscow, Kremlin spokesman Dmitry Peskov described the allegation against Kyiv as "disturbing" and "a matter of very deep concern".
There were no immediate reports of deaths, and clashes involving artillery and sniper fire are common along the frontline, but any significant increase in fighting could be the spark to ignite a wider conflict.
The United States has claimed Moscow could be looking for a pretext to invade and earlier this week Russian President Vladimir Putin claimed that Kyiv was committing "genocide" in the eastern Donbas region.
US Defense Secretary Lloyd Austin described Thursday's reports as "troubling".
"We've said for some time that the Russians might do something like this in order to justify a military conflict. So we'll be watching this very closely," Austin told journalists after a meeting with NATO counterparts.
On her way to Kyiv Thursday for talks, British Foreign Secretary Liz Truss said claims of increased Ukrainian military activity on the front were "a blatant attempt by the Russian government to fabricate pretexts for invasion".
"This is straight out of the Kremlin playbook," she said.
Western officials say Russia has amassed well over 100,000 troops and significant military hardware near Ukraine's borders in preparation for a potential invasion, which Washington says could take place "at any time".
Russia has said "large-scale" military exercises are taking place in various areas, including near Ukraine, but has not provided any specific numbers and has repeatedly denied any plans for an attack.
Moscow has made several announcements of troop withdrawals this week and on Thursday said that units of the southern and western military districts, including tank units, had begun returning to their bases from near Ukraine.
Defence ministry spokesman Igor Konashenkov said some troops had returned to their garrisons in several areas far from the border, including Chechnya and Dagestan in the North Caucasus, and near Nizhny Novgorod, some 300 kilometres (185 miles) east of Moscow.
After previously announced withdrawals earlier this week, the United States, NATO and Ukraine all said they had seen no evidence of a pullback, with Washington saying Russia had in fact moved 7,000 more troops near the border.
"This is a process that will take some time," Kremlin spokesman Peskov said.
Russian officials have accused the West of provoking "hysteria" with claims of a planned invasion.
At the border of Ukraine and Belarus — where Russian and Belarusian forces are carrying out major joint exercises — residents feared being caught in the middle of a clash of great powers.
"The Ukrainians could start something, thanks to the Americans and the British, who brought all their weapons here," 87-year-old Lidiya Silina told AFP in her green wooden shack close to the border.
"For them, Ukraine is just a field of battle with Russia."
The joint drills in Belarus — which the US says involves some 30,000 Russian troops — are set to end on Sunday.
Russia insists its forces will go back to bases after the exercises so a significant withdrawal early next week could set the stage for the crisis to ease.
Belarusian leader Alexander Lukashenko, who will be in Moscow for talks with Putin on Friday, said his country could host nuclear weapons if it faces any external threats.
Russia has blamed the West for provoking the tensions, saying Washington and its European allies have for too long ignored Moscow's security concerns on its doorstep.
Putin has demanded that Ukraine be forever banned from fulfilling its hopes of joining NATO and for the alliance to roll back its deployments near Russia's borders.
Russian Foreign Minister Sergei Lavrov said Thursday that Moscow would send a reply to US proposals about European security later in the day.
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