Violence escalated in eastern Ukraine on Thursday, with Russian-backed separatists and authorities in Kyiv trading accusations over cease-fire violations along the front line separating the two sides as Western governments said Moscow continued to mass troops on the borders of its smaller neighbor.

A kindergarten and a school in Ukrainian-held towns were hit by mortar shells, according to the Ukrainian army and local residents. Authorities in separatist areas said mortar attacks had also damaged several buildings in towns...

Violence escalated in eastern Ukraine on Thursday, with Russian-backed separatists and authorities in Kyiv trading accusations over cease-fire violations along the front line separating the two sides as Western governments said Moscow continued to mass troops on the borders of its smaller neighbor.

A kindergarten and a school in Ukrainian-held towns were hit by mortar shells, according to the Ukrainian army and local residents. Authorities in separatist areas said mortar attacks had also damaged several buildings in towns there. No fatalities were reported on either side.

Ukrainian President Volodymyr Zelensky, who was visiting front-line troops, described the damage to the schools as a “big provocation” by pro-Russian forces.

In Moscow, Kremlin spokesman Dmitry Peskov was quoted by Russian state news agency TASS blaming Kyiv for increasing tensions and saying, “It’s clear that the situation in the Donbas is ramping up.”

“The situation at the borders of Russia may ignite at any moment,” he said.

Western officials dismissed Russia’s charges.

“Reports of alleged abnormal military activity by Ukraine in Donbas are a blatant attempt by the Russian government to fabricate pretexts for invasion,” U.K. Foreign Secretary Liz Truss said in a tweet. “This is straight out of the Kremlin playbook.”

A Ukrainian soldier on Thursday in Stanytsia Luhanska, a town in the east of the Ukrainian-controlled Luhansk region across a bridge from the region’s Russian-held capital city.

Photo: aleksey filippov/Agence France-Presse/Getty Images

Russian Foreign Minister Sergei Lavrov denied that, saying: “Attempts to shift all the blame for what is happening around Ukraine onto Russia will not succeed.”

Russia was expected to raise the issue of conflict in eastern Ukraine on Thursday at the United Nations, where Moscow has filed a report that alleges Ukraine’s military is pursuing the “genocide of the Russian-speaking population,” according to documents reviewed by The Wall Street Journal.

Russia has made its genocide assertion repeatedly. Ukraine denies it and blames Moscow for igniting a conflict in the area—known as the Donbas—that has killed some 14,000 people since 2014 and forced millions of residents to flee to other parts of Ukraine, Russia or the West.

Exchanges between Ukrainian forces and Russian-armed fighters in the Donbas’s Donetsk and Luhansk regions have occurred regularly in the years since conflict began there in 2014, despite a cease-fire agreed to a year later.

Organization for Security and Cooperation in Europe monitors document cease-fire violations, work that has become more challenging after the U.S. and the U.K. pulled their staff from the mission in recent days. Mr. Zelensky said Thursday that the recent uptick in violence showed the need for these monitors to return.

Officials in Kyiv and the West have warned for weeks that Moscow could use fighting in the Donbas as a reason to move into eastern Ukraine.

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Russian President

Vladimir Putin earlier this week accused Kyiv of perpetrating “genocide” against the mostly Russian-speaking population of the Donbas, as the Russian parliament has urged him to recognize the two self-proclaimed republics.

Russia has deployed 150,000 heavily armed troops, according to U.S. estimates, effectively surrounding Ukraine on three sides, as it carries out military exercises in Belarus and the Black Sea.

The Kremlin has sent mixed signals in recent days, announcing the drawdown of some of its troops and suggesting Moscow could be open to negotiations with the West to defuse the crisis and address what it says are threats from Western powers to Russia’s security.

Cease-fire monitors inspected damage from shelling in Vrubivka, in eastern Ukraine, on Thursday.

Photo: Vadim Ghirda/Associated Press

Ukrainian and Western officials say Russia instead has continued its military buildup around Ukraine. Ukraine on Wednesday raised the alert status of its armed forces and said it had distributed more ammunition to troops on the front lines.

Shelling on Thursday in Stanytsia Luhanska, a town in the east of the Ukrainian-controlled Luhansk region just across the bridge from the region’s Russian-held capital city, was the worst since 2015, said local entrepreneur Aleksey Chernikov.

“Many locals are in panic, some are trying to flee, others are trying to hide,” he said. Power was knocked out by the shelling but then returned, he said.

A shell hit the local kindergarten just after 9 a.m., Stanytsia Luhanska residents said. Some 20 children were in a corridor in a different part of the building at the time, all dressed up and ready for a walk, and weren’t hurt, said Ekaterina Artemova, a local nongovernment-organization worker.

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Three kindergarten staff were treated for concussion, officials said. Images shared by local residents showed a recreation room with a gaping hole in a brick wall. Debris was strewn on the ground, amid soccer balls and a wall painting of a blue elephant and a parrot.

Albert Zinchenko, head of the local civilian-military administration, said he thought pro-Russian militants hit the kindergarten by mistake as they targeted a nearby railway depot.

The only crossing point that operates daily between Russian-held areas of the Donbas and the rest of Ukraine is located in Stanytsia Luhanska. Some 3,000 people cross it daily.

In another town along the front line, Vrubivka, a shell Thursday morning hit the courtyard of an elementary school, according to local residents reached by phone.

“All the children were terrified,” said Olena Makarenko, the grandmother of a first-grader at the school. Ms. Makarenko, 44, said that as she sheltered in a neighbor’s home with her other grandson a piece of shrapnel flew through the window. It was the worst violence in Vrubivka since 2018, she said.

Yan Leshchenko, head of the Luhansk People’s Militia in the Russian-backed part of Luhansk, said Kyiv manufactured the attack on the kindergarten.

“In order to justify the criminal actions of the enemy, the Ukrainian side carries out stuffing and publishes staged materials about the destruction of civilian infrastructure, allegedly as a result of the fire activity of the People’s Militia,” he said in a post on the pro-Russian Luhansk authorities’ official website.

In Luhansk city, seven residential buildings as well as a gas pipeline and power line were damaged by shelling from Ukrainian-controlled areas, authorities there said.

Viktor Pris, a representative for pro-Russian authorities in Donetsk, said on Telegram messenger that “the situation on the line of contact has sharply escalated” with more shelling than had been recorded since April 2021. “The enemy is making attempts to unleash active hostilities,” he said.

—William Mauldin in Washington contributed to this article.

Write to Yaroslav Trofimov at yaroslav.trofimov@wsj.com and Ann M. Simmons at ann.simmons@wsj.com