AVDIIVKA (NYTIMES) - Spartak Kyurdzhiev was bragging to friends on Wednesday (April 20) that he never hides from bombings anymore when a volley of rocket fire landed with such intensity nearby that he dashed into the school he was standing in front of.

"Let's go!" Spartak, 16, shouted at his two friends, and they ran for cover.

The residents of Avdiivka, in Ukraine's eastern Donbas region, have been living for years in the shadow of the fight against Russian-backed separatists. But the peril they are facing now, with the Russian military massing just outside town in a new and potentially more lethal phase of the war, could be far more devastating.

Avdiivka is no longer simply a town on the front lines of the conflict with the separatists, absorbing the periodic volleys of a simmering eight-year war. It is now a significant impediment to Moscow's military goals - squarely in the path of Russian forces aiming to advance and gain control in the east.

The town's hardened force of Ukrainian fighters is dug into an extensive World War I-style trench system and will be difficult to dislodge without massive firepower. Whether Avdiivka and towns like it in the Donbas can repel the Russian forces will determine whether Moscow can claim a narrower victory after being soundly defeated in the north.

But the Kremlin is intent on wresting the eastern territory from Ukrainian control, and residents in Avdiivka and all along the so-called line of contact with separatist territory have begun to get a taste of what Russia's military has in store.

In Avdiivka, artillery shelling has intensified, bolstered by airstrikes this week that destroyed a supermarket and athletics store right in the middle of town, according to local officials. Dozens have been wounded in recent weeks, and each week brings a handful of civilian deaths.

Already there are signs of a more costly engagement with the Russian forces. The lone surgeon at the local hospital, Dr. Mikhail Orlov, said the injuries he has had to treat in recent weeks are more serious than anything he has seen since the separatist fight began in 2014. He showed off a foot-long piece of metal shrapnel from a rocket that he said he removed from a woman's back last month. She survived.

"The wounds are much deeper, with trauma that involves chunks of muscle mass ripped away," he said.

Life in town is miserable. There is no heat or running water, and the electricity is spotty at best. As many as 2,000 people have taken up residence in the city's 60 bomb shelters, and many more have fled, said Vitaliy Barabash, head of Avdiivka's military administration. But so far, the Russian forces have not yet broken through the Ukrainian defensive position, he said.

"They can't get through the front lines and so they're starting simply to destroy the city," Barabash said of the Russian forces.

He likened the bombing of Avdiivka to the early days of the attack on Mariupol, the Ukrainian port city that Russian forces have turned into a charred ruin.

The story is similar throughout the east. This week, Russia's foreign minister announced that his country's rocket and artillery forces had struck hundreds of military targets, the opening salvo in this phase of President Vladimir Putin's war against Ukraine, which is about to enter its third month.

From the northern city of Kharkiv to Mariupol in the south, Russian forces are arrayed along a front that stretches nearly 480km, preparing to try to seize the Donbas.

While fighting in the extreme northern and southern portions of the front has been intense for weeks, Russian forces have recently begun to probe points along the middle, including the cities of Izium and Kramatorsk, where a bombing this month at a central train station killed at least 50 people, and Avdiivka.

Together, they make up a chain of cities fortified by the Ukrainians that stand in the way of Russia's new military operation. For the Russians to conquer the Donbas, these cities must either capitulate or meet the fate of Mariupol, which will barely be habitable once the Russian military is through with it.

On Wednesday, Avdiivka was nearly in full bloom, with pink flowering apple and cherry trees and tulips in just about every yard, although few people remain to enjoy it. Only about 6,000 of the city's 30,000 pre-war residents remain, officials said, and the periodic boom of artillery and rocket fire throughout the day kept most people underground.

The central hospital in Avdiivka has been newly renovated, but it had no power on Wednesday and water for drinking is available only from a large blue cistern in the lobby. It is operating with a skeleton crew of 40 people. The medical director and Orlov, the surgeon, have been living on the premises for more than a month.

"If I go home I may not be able to return," Orlov said. "I may come under fire along the way." His colleague, Vitaliy Sytnik, the medical director, said there would have been far more injuries had residents not adopted the habit of spending much of their time in basements.

A dank storage space lit by a single candle is where Valentina Mutyeva, 72, has spent much of the past month, along with 10 other people, including her daughter and two grandsons. Although the younger people often travel up to the surface, where much of the cooking is done on a wood-fired stove in the courtyard, Mutyeva said she spends most of her days below ground.

"You go up just for five minutes and they start to shell," she said. "And at night they shell." While she said she laments the comforts of home, what really concerns her is the effect the war has had on the town's children. She pointed to one of her grandsons, Sasha, a slight blond boy of 15, who she said has been deeply scarred by the fighting that has raged for much of his life.

"He started to walk around at night and talk to himself because of this war," she said through tears. "Children of the underground. It's so brutal."

In another part of town, a rocket blast that rattled the walls of a basement housing about 30 people somehow didn't faze a 6-year-old girl named Varvara, who sat drawing at a little table.

When she was finished, she showed a reporter a picture of a green alien she had drawn, with a vacant black eye that she said was for seeing the future. She happily announced that the alien had told her the reporter would live forever.

"What about the war, when will it end?'' she was asked.

"That he cannot see," she said.