A Ukrainian commander describes the grueling battle against dug-in Russian forces.

Even as he commanded troops in a fierce, weeklong fight to capture one Russian position, Col. Viktor Sikoza received disturbing news: The Russians were using this time to build yet another fortification behind it.

“They are still building their defenses,” Colonel Sikoza said. “They carry on doing it” even as Ukraine is pressing forward, however slowly, in a high-stakes counteroffensive in the country’s south.

The troops of Colonel Sikoza, the commander of 36th Marine Brigade, have been the tip of the spear in Ukraine’s push and have advanced about five miles into a bulge into Russian lines in southern Ukraine.

Colonel Sikoza is just one commander, but his account matches Ukrainian reports of heavily entrenched Russian positions. Over the past week, Colonel Sikoza oversaw an assault on a forest that was partly surrounded by swampy, low-lying terrain. Russian forces had dug in and mined the only ground around that was firm enough to support armored vehicles. The assault, he said, had to take place on foot.

Infantry filtered into the forest and fought at close range, he said. “We are marines — we are aggressive,” he said. A company of Russian soldiers, about 80 men, were dug into trenches in the trees, he said. Colonel Sikoza described how a decisive turn came when his troops captured two bunkers and a trench line on the edge of the forest, partly cutting off the Russians ability to resupply the group and forcing them to retreat.

His soldiers are highly motivated to capture positions, and with them Russian prisoners: In the first month of the war, the 36th Marine Brigade was surrounded in the city of Mariupol and more than 1,000 marines were taken prisoner by the Russians. “We want to trade them for our guys,” Colonel Sikoza said of Russian captives.

Colonel Sikoza’s advance to the south is also a personal odyssey. He escaped from the Crimean Peninsula when Russia occupied it in 2014. If the counteroffensive is successful, it could put Ukrainian artillery within range to threaten the isthmus to the peninsula, cutting Russian supply lines.

But it has been painfully slow. President Volodymyr Zelensky has conceded that the counteroffensive is not going as rapidly as some allies had hoped, and American officials have said Ukraine is losing Western-provided armored vehicles in the minefields.

“For more than a year, the enemy fortified here,” Colonel Sikoza said in an interview at a picnic table in the shade of a walnut tree in the yard of his command post near the front. Every minute or so, the booms of outgoing and incoming artillery rang out.

Of the Ukrainian effort to advance, he acknowledged: “It will not go according to the pace we counted on.”

Ahead of the Ukrainian bulge in this spot, the Russians have completed a third line of defenses, Colonel Sikoza said. They have laid additional concrete tank barriers of a type the Ukrainians call dragon’s teeth. And they have deployed additional troops.

There is little he can do about it, he said. “Unfortunately, we do not have enough precision weaponry to hit targets at long range” behind the Russian front lines. The additional defenses, he said, will slow Ukraine further.

Still, Colonel Sikoza’s troops have scored some successes.

One soldier, who asked to be identified only by his rank and first name for security reasons, Lieutenant Yevhen, said he had shot down a Russian attack helicopter using a Javelin anti-tank guided missile, a rare feat with a weapon mostly intended to hit targets on the ground.

And though the brigade operates mostly Soviet-legacy artillery systems, it was able to hit a Russian barracks far behind the front line, Ukrainian officers said. A group of Russian soldiers had posted a video on social media complaining of poor living conditions and what they said were unreasonable orders from commanders.

Lt. Denys Ryabynko, who commands a unit of Grad rocket artillery, was interested less in the complaint than in the distinctive brick building in the background. The Ukrainians were able to identify it in a village behind Russian lines and hit it with a barrage of rockets, he said.

Yurii Shyvala contributed reporting.