At the front in eastern Ukraine, distant artillery duels mix with tank assaults and vicious firefights.

The impact of a tank round cracked the bunker’s plaster roof and sent uniformed men scrambling. Flak jackets and helmets were flung on and automatic weapons cocked. Amid a crescendo of machine gun fire, a tall soldier slung an anti-tank missile launcher over one shoulder and took a slow drag on his cigarette.

The Russians were close.

Fighting in eastern Ukraine has mostly occurred at a distance, with Ukrainian and Russian forces lobbing artillery at one another, sometimes from dozens of miles away. But at some points along the zigzagging eastern front, the combat becomes a vicious and intimate dance, granting enemies fleeting glimpses of one another as they jockey for command of hills and makeshift redoubts in towns and villages blasted apart by shells.

On Wednesday, one such dance played out as a Russian unit of about 10 men entered the village where soldiers from a Ukrainian contingent, the Carpathian Sich Battalion, had dug in. The Russian troops were likely helping target incoming tankfire, including the round that jolted the Ukrainian soldiers into action. Ukrainian forces spotted the Russian soldiers and opened fire, pushing them back.

“It was a sabotage group, intelligence,” said a 30-year-old fighter with the call sign Warsaw, panting after the brief firefight. “Our guys were not asleep and reacted quickly, forcing the enemy to flee.”

So it goes every day, every hour for the fighters of the Carpathian Sich Battalion, a volunteer unit named for the army of a short-lived independent Ukrainian state created just before World War II. Attached to the Ukrainian Army’s 93rd Mechanized Brigade, the battalion is deployed along a line of villages and trenched farmland in the Kharkiv region, tasked with holding back Russian forces pushing down from their stronghold in the occupied Ukrainian city of Izium.

The battalion gave a reporter and a photographer with The New York Times permission to visit a frontline position on condition that the precise location of their base not be revealed. Most soldiers agreed to identify themselves only by their call signs.

They have not faced an easy fight.

The Russian military has deployed a massive force along this front in eastern Ukraine, bringing its overwhelming superiority in tanks, fighter jets, helicopters and heavy artillery to bear.

The massive war machines rarely remain quiet for long. Tanks in particular have become a serious menace, fighters said, often coming within a mile of the battalion’s positions and wreaking absolute havoc. Already this month, 13 soldiers with the battalion have been killed and more than 60 were wounded.

“It’s a completely different war than I’ve seen in places like Afghanistan or Iraq,” said a colonel named Mikhailo. “It’s heavy fighting. Nobody cares about the law of war. They shell little towns, use prohibited artillery.”

Many of the battalion’s soldiers had experience in the eight-year war against Russian-backed separatists in eastern Ukraine, and had seen fighting in some of the conflict’s most intense battles. But most had been settled into civilian life for years.

One tall, bearded soldier with the call sign Rusin owns a business selling bathtubs in the mountainous region of Transcarpathia, in western Ukraine. But when Russia invaded on Feb. 24, he quickly married his girlfriend — he said he wanted someone waiting for him back home — and headed to war filled with a sense of mission.

“We understand that this is not a war between Ukraine and Russia,” he said. “This is a war of the pure and light that exists on this earth and darkness. Either we stop this horde and the world gets better, or the world is filled with the anarchy that occurs wherever there is war.”