ZAPORIZHZHIA REGION, Ukraine — Under the cover of darkness, leaning forward under the weight of packs and rifles, a squad of soldiers walked along a muddy lane and slipped into a village house.
They were Ukrainian infantrymen of the 117th Separate Mechanized Brigade, assembling for a last briefing and roll call several miles from Russia positions before heading to the trenches on the front line. Stolid men in helmets and rubber boots, they listened in silence as an intelligence officer briefed them on a new route in to their positions.
“Morale is all right,” said the deputy battalion commander, who uses the call sign Shira, standing nearby to see the men off. “But physically we are exhausted.”
Ukrainian troops along most of the 600-mile front line are officially in defensive mode. Only in the southern region of Kherson are they still on the offensive in a tough assault across the Dnipro River.
But the fighting has not eased and Russian forces are now on the offensive.
The capture of the town of Robotyne in the southeastern Zaporizhzhia region was far as Ukrainian troops managed to advance in their summer counteroffensive. No breakthrough occurred. Now, in the trenches around Robotyne, Russian units are attacking daily. Ukrainian troops try to counterattack immediately if they lose ground, commanders said.
“It is something like a game of Ping-Pong,” said a Ukrainian National Guard platoon commander who uses the call sign Planshet, meaning “tablet.” “There is a portion of 100 to 200 meters of ground always being taken and retaken,” he said.
Indeed, Ukrainian soldiers and commanders interviewed in recent weeks along a broad stretch of the central and eastern front said that Russian attacks were so intense that operating near the frontline has never been so dangerous.
Russia has in recent days turned its focus to bombing Ukraine’s big cities to wear down civilians; for weeks its ground forces have been mounting attacks to claw back territory lost last summer and to seize long-prized Ukrainian redoubts along the eastern front.
Well accustomed to Russian artillery fire, soldiers said that since March they had suffered the additional devastating power of glide bombs, half-ton explosives unleashed from planes that smash through underground bunkers.
“They would send them two by two by two, eight in an hour,” said a 27-year-old soldier known as Kit, of the 14th Chervona Kalyna National Guard Brigade. Like others interviewed, Kit identified himself by his call sign, according to military protocol. “It sounds like a jet coming down on you,” he said, “like hell’s gate.”
The destruction wrought by glide bombs is visible in towns and villages near the front line. The town of Orikhiv, about 12 miles north of Robotyne, once served as a command center for the counteroffensive. Now it is an empty shell, the main street deserted, the school and other buildings split asunder by massive bomb craters.
A lone workman, Valera, was riding a bicycle through the town. He said he had stayed despite the heavy bombardment because he had paid work, fixing generators. He lived off humanitarian aid and was feeding 20 stray cats at his home, he said.
Soldiers moved cautiously in the area, mostly living in basements and staying undercover, out of sight.
That is because the latest menace is Russia’s use of F.P.V. kamikaze drones, which has forced Ukrainian soldiers largely to abandon vehicles in frontline areas and operate on foot.
A cheap commercial drone, the F.P.V. — for first person view — has become the latest weapon of the moment in the Ukrainian war. It can fly as fast as a car, carries a lethal load of explosives and is guided to its target by a soldier sitting in a bunker several miles away.
Both the Russian and Ukrainian armies are using them to hunt and attack targets because they cut out the delay of relaying back coordinates and requesting artillery strikes. Ukrainian soldiers said they often use the drones instead of artillery because shells were increasingly in short supply and the drones are a cheap, quick weapon for attacks on nearby Russian vehicles, bunkers and infantry.
Military units from both sides post videos online of their successful strikes, which end with a scrambled black screen at the moment of detonation. Several Ukrainian drone units allowed journalists from The New York Times to watch live operations from positions near the front line as they tracked Russian soldiers and attacked selected targets.
One unit showed videos of a hit that destroyed Russian surveillance cameras and an antenna on an office building. Another targeted a Russian bunker in a tree line, although the drone was deflected by Russian electronic jamming before impact.
Only one in several drones hits its target, and many are lost to jamming and other interference, soldiers said.
For those on the receiving end of F.P.V. drones, defending and supplying the front line have become increasingly risky.
“It is extremely dangerous to go by car,” said a Ukrainian National Guardsman, who uses the call sign Varvar. Men of his unit said that since September they had been leaving their armored vehicles and walking in six miles to positions. “You can only go in on foot,” Varvar said.
The men of the 117th Brigade, who were deploying to the front line in the Zaporizhzhia region on a recent night, faced a four-mile hike through rain and mud, the intelligence commander said. If they were wounded and captured, Russian troops would execute them, he warned them.
The long, arduous slog to carry in ammunition and food to supply troops and to carry out the wounded was one reason Ukraine could not sustain its counteroffensive, a company commander, Adolf, 23, said.
Ambulances and supply vehicles came under fire from kamikaze drones so often that his unit stopped using them, resorting instead to a four-wheeled buggy that volunteer engineers rigged up to carry a stretcher. The buggy was hidden under some trees beside his command post several miles from the front line.
Ukrainian units are dealing out the same treatment with F.P.V. drones on Russian lines and say they were the first to start using drones to attack targets. But the Russians have copied the tactic and flooded frontline areas with drones in recent weeks, to lethal effect, Ukrainian soldiers and commanders said.
“My impression is Russia is interested in drones at the state level,” the soldier known as Kit said, but in contrast, Ukraine still largely relied on volunteers and civilian donors for its drone program. “My sense,” he said, “is the government should be doing more.”
The Russians were employing subterfuge as well, Planshet said, playing tapes of gunfire on drones to make Ukrainian soldiers think they were under attack, leave the bunkers and reveal their positions.
Some members of his platoon said the Russians used drones to drop smoke grenades into their trenches. One soldier, who uses the call sign Medic, said it seemed like a kind of tear gas.
“It causes a very strong pain in the eyes and a fire, like a piece of coal, in your throat and you cannot breathe,” he said.
Several soldiers donned gas masks to treat the men affected, but when two men in the platoon crawled from the bunker to flee the gas, they were killed by grenades dropped from Russian drones hovering above, soldiers said.
The toll is heavy for all units along the front. Almost everyone has been wounded or survived a narrow escape in recent months, soldiers said.
“We are short of people,” said an intelligence commander of the 117th Brigade who uses the call sign Banderas, after the actor. “We have weapons but not enough men.”
Yet many remain optimistic. Farther east in the Donetsk region, Maj. Serhii Betz, a battalion commander of the 72nd Separate Mechanized Brigade, set out before dawn on a recent day, driving down muddy roads rutted with ice to check on his drone units close to the front line. He invited New York Times journalists along.
The teams work underground, in bunkers lined with tree trunks and covered with earth. On a computer monitor, the commander switched on a livestream drone feed from a neighboring brigade where a battle was unfolding.
“Russian tanks entering the village,” a commander said over a walkie-talkie. “Is everything ready?” the major asked the drone team. “A tank is a cool target to destroy; let’s help our brothers.”
Mice scurried through their bunker, rustling in a rubbish bag, as the newly deployed team, fresh from training, fiddled with wiring and switches to get an F.P.V. airborne over the Russians’ positions for their first strike.
They were too slow, and their first two flights crashed, downed by Russian electronic jamming.
But the major was satisfied. “We are developing,” he said.
Olha Konovalova contributed reporting from the Zaporizhzhia region, and Christiaan Triebert from Auriac-du-Périgord, France.