STANYTSIA LUHANSKA, Ukraine — The sharp cracks of explosions echoed off buildings and flashes of light from incoming artillery shells silhouetted trees on the edge of this town on the frontline of the war in eastern Ukraine, which escalated sharply on Thursday.
The fighting between Russian-backed separatists and Ukrainian government forces has been flaring for eight years, long before the threat of a broader conflict between Russia and Ukraine that has loomed for the last month. Daily skirmishes, mostly low-level and localized, had become routine.
But an outbreak of hostilities on Thursday, which each side blamed on the other, was viewed in Ukraine and in Western capitals as a particularly perilous moment for its potential to spiral into a bigger conflict that would draw the United States and Europe into a tense standoff with Russia.
The United States has said that Russia has massed about 150,000 troops on Ukraine’s border. And Western military analysts have predicted that Russia may claim an unprovoked attack, perhaps manufactured by Moscow, to justify an intervention in eastern Ukraine, possibly under the claim of serving as a peacekeeping force.
The artillery strikes began early Thursday and continued into the evening. The Ukrainian military reported 47 cease-fire violations in at least 25 different locations, including two towns, Stanytsia Luhanska and Popasna.
The Ukrainian military said shells hit a kindergarten, wounding three teachers but no students, as well as the playground of a high school. They also said two soldiers and a woman at a bus station were wounded. There were no reported fatalities.
“It was a whistling sound, then an explosion,” said Tatyana Podikay, the director of the school, called Fairytale Kindergarten.
The teachers herded the students into a hallway with no windows, the building’s safest place, and waited for parents to pick them up. “To create a calm psychological atmosphere the teachers told stories, and whoever needed it got a hug,” Ms. Podikay said.
Analysts said the nature of the shelling, which hit multiple sites along the contact line all in a single day, was unusual compared to recent months. “Today it was long-distance and synchronized shelling,” said Maria Zolkina, a political analyst. “It was simultaneous. This is notable.”
After a lull in the afternoon, artillery fire resumed Thursday evening in Stanytsia Luhanska, a hardscrabble town of dusty, potholed roads surrounded by farm fields, not far from the Russian border.
Shells exploded in or near the town in at least two volleys of a half dozen rounds each. Drivers stopped their cars, got out and listened, worriedly.
Amid the fighting, President Volodymy Zelensky of Ukraine flew to the front line to visit troops and was quoted in Ukrainian media saying he was proud of the army for “giving a worthy rebuff to the enemy.”
In Brussels, the U.S. defense secretary, Lloyd J. Austin III, said that the reports of shelling were “troubling.” While the United States was still gathering details, Mr. Austin said: “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.”
Understand the Escalating Tensions Over Ukraine
A brewing conflict. Antagonism between Ukraine and Russia has been simmering since 2014, when the Russian military crossed into Ukrainian territory, annexing Crimea and whipping up a rebellion in the east. A tenuous cease-fire was reached in 2015, but peace has been elusive.
Ukraine’s foreign minister, Dmytro Kuleba, blamed Russia for a “severe violation” of the tenuous cease-fire agreement in the region, while President Zelensky described it as “provocative shelling.”
The Kremlin was taking a different line. “We have warned many times that excessive concentration of Ukrainian forces near the contact line, together with possible provocations, can pose terrible danger,” President Vladimir V. Putin’s spokesman, Dmitri S. Peskov, said. He added that he hoped Western countries would warn Kyiv against a “further escalation of tensions.”
The Russian-backed separatists also blamed the Ukrainian army. Leonid Pasechnik, head of the self-proclaimed Luhansk People’s Republic, said the Ukrainian army had shelled civilians early this morning — a claim that could not be independently verified.
Russia’s foreign minister, Sergey V. Lavrov, has said about a quarter of the inhabitants in the separatist regions — that would be 750,000 out of about three million — are Russian citizens. A strike that wounds or kills a Russian citizen could elevate the risk of a Russian response.
To highlight what it called reckless firing into civilian areas, the Ukrainian military flew reporters to the site of the damaged kindergarten. The strike also knocked out electricity and sent residents scrambling into basements to seek cover.
The Ukrainian military said a 122-millimeter artillery shell hit the school, spraying cinder blocks into a play area for toddlers that was empty at the time.
Artillery and small-arms fire are common along the frontline, where an international monitoring group typically reports dozens to hundreds of cease-fire violations every day in recent years. Homes, schools, administrative buildings and infrastructure including electrical pylons are often damaged. Earlier this year, Ukrainian authorities reported that a drone strike hit an abandoned school in an eastern Ukrainian town.
Andrew E. Kramer reported from Stanytsia Luhanksa, Ukraine, and Valerie Hopkins from Kyiv. Maria Varenikova contributed reporting from Kyiv, and Ivan Nechepurenko from Moscow.
