ZAPORIZHZHIA REGION, Ukraine — In a painstakingly slow process that has come to define the speed of the Ukrainian counteroffensive, small groups of sappers on the front lines are crawling across minefields – sometimes literally on their stomachs – to detonate Russia’s defenses and clear a path for troops to advance.

The long buildup to the counteroffensive, which began about a month ago across multiple segments of the battlefield in the country’s east and south, gave the Russians time to prepare, soldiers said. Areas are between 3 and 10 miles deep in front of the Russians’ main strongholds have been densely mined with antitank and antipersonnel mines and trip wires. These defenses have been successful in stalling the Ukrainian advance, they said.

As a result, Kyiv’s forces have changed strategy, Ukrainian military personnel said. Rather than try to break through with the infantry fighting vehicles and battle tanks that Western allies provided to aid Ukraine in this counteroffensive, units are moving forward, slowly, on foot.

“You can no longer do anything with just a tank with some armor because the minefield is too deep, and sooner or later, it will stop and then it will be destroyed by concentrated fire,” Gen. Valery Zaluzhny, Ukraine’s military chief, said recently in an interview with The Washington Post.

Ukraine’s struggles on minefields have exposed vulnerabilities of the personnel carriers and tanks – especially the newly arrived American Bradley fighting vehicles and German Leopard tanks – that officials had hailed as being key for Ukraine to seize back occupied territory from the Russians. The vehicles have won praise from soldiers – even after they’ve hit mines, most people inside survive with just minor injuries – but they have not been able to breach Russia’s defenses alone. Zaluzhny has said modern fighter jets, such as the U.S.-made F-16, and other systems are needed to better support ground operations.

“We need special equipment, we need special remote mine-clearance equipment,” Zaluzhny said, adding that Ukraine is using U.S.-provided M58 Mine Clearing Line Charge (MICLIC) systems but that “they are also being destroyed, yes. There’s nothing wrong with that. It takes a lot of them.”

In an address late Friday, President Volodymyr Zelensky acknowledged the difficulty of advancing. “We must all understand very clearly – as clearly as possible – that the Russian forces on our southern and eastern lands are investing everything they can to stop our warriors, he said. “And every thousand meters of advance, every success of each of our combat brigades deserves gratitude.”

A senior Ukrainian official, who, like others, spoke on the condition of anonymity to discuss sensitive military matters, said Kyiv received less than 15% of the quantity of demining and engineering materiel, including MICLICs, that it asked for from Western partners ahead of the counter-offensive. Some of that equipment arrived just last week, the official said.

Defense Minister Oleksii Reznikov and Zaluzhny told The Post that they have informed their Western counterparts that they urgently need more mine-clearing systems, such as Bangalore torpedo explosive charges. Ukraine has held back some of the brigades and Western weapons prepared for the counteroffensive as it attempts to penetrate the minefields.

Gen. Mark A. Milley, chairman of the Joint Chiefs of Staff, “opens up maps to me and says, ‘Look, there’s nothing here, there’s nothing here, there’s nothing here anymore,'” Zaluzhny said. “Minefields are one of the problems that certainly affects the pace of the offensive. This is a problem we see. Could it have been resolved more quickly? It could have been. How could it be solved? At least General Milley knows. The other question is, can he help with that? I don’t know.”

U.S. officials said that they have provided Ukraine with nearly every type of equipment it requested ahead of the counter-offensive. Officials cautioned that it is not always possible to provide the quantities Ukraine asks for, but said that with the MICLIC systems specifically, Washington is working to soon provide more of not only the system but also the charges it uses to detonate a long row of mines.

The officials added that the U.S. decision to provide Ukraine with controversial cluster munitions will give Kyiv fire superiority for the first time in this conflict, allowing the Ukrainians the proper time and space to use the engineering equipment they already have.

Ukrainian military personnel on the ground also described hesitation to use the larger, more advanced demining equipment. Because, in the Ukrainians’ opinion, there are so few mine-clearing systems, they have become an easier target for Russian forces, which have prioritized striking them. The depth and density of the minefields are particular challenges along the southern Zaporizhzhia front line, where the Russians widely expected the Ukrainians to assault and attempt to sever a land corridor across the occupied region that connects the Russian border to Crimea, the Ukrainian peninsula that Moscow illegally annexed in 2014.

The area’s terrain is largely sprawling, with open fields with few places for the Ukrainians to camouflage their larger equipment and vehicles. And the Russians chose the high ground for their positions, soldiers said.

A commander – The Post is identifying him by his call sign, Oskar, by Ukrainian military protocol – with an engineering and sapper unit in Ukraine’s 47th Mechanized Brigade said his group received a German-provided Wisent mine-clearance tank that it used ahead of the counteroffensive’s start in the Zaporizhzhia region. The tank and similar Soviet models successfully cleared some pathways for the brigade’s units to make their first push.

“But now their use is already ineffective because the enemy expects the appearance of such equipment, which is massive, which is noisy, which is easy to see and, accordingly, to strike,” Oskar said.

Another officer in the 47th brigade said that on the counteroffensive’s first day, some of the brigade’s units, riding in Bradley fighting vehicles and Leopard battle tanks, mistakenly took a wrong route, into a minefield, instead of one that had been prepared by sappers in advance.

Obstacle-clearing vehicles were at the front of the columns, but the group was forced to stop when vehicles in the rear unexpectedly ran into mines and got trapped. The chaos created a cluster of vehicles in one spot. The Russians then started to attack the Ukrainians from helicopters overhead and with antitank missiles, badly damaging or destroying several of the personnel carriers and tanks. Some units that left their equipment behind still managed to seize Russian trench positions, according to the Ukrainian officer.

“When the enemy sees even a Leopard tank in front of him and special engineering equipment, he will destroy the special equipment first,” he said. “Because without it, all the others will not pass. And in just a couple of days of the offensive, several such vehicles were destroyed along with their crews.”

Because the Russians have drones in the sky on the lookout for any mine-clearance systems to target with artillery and missiles, the Ukrainians are trying for now to save the few they do have by doing the job manually. Sapper units – sometimes a group of just four people – will often wait for twilight to clear paths, as they are too visible in the daylight and can be seen through night-vision devices in the dark.

Walking with a metal detector is unrealistic, sappers said, because they are too visible. So they crawl, relying on their vision to spot mines.

“It slows us down a lot, because the work of a sapper, it needs time and tranquility,” said Lt. Col. Mykola Moroz, the commander of the 128th Mountain Assault Brigade’s engineering and sapper battalion. “It’s not possible to do our work in these circumstances.”

The Russians are also able to drop more mines from drones, reseeding areas that the Ukrainians had cleared. And once the Ukrainians get to a Russian trench line and seize the new position, that might be mined, too. Also, because they’re moving on foot instead of on their new Western vehicles, soldiers said that replenishing ammunition supplies and evacuating wounded is more challenging.

“We were preparing, but the Russians were also preparing,” said Reznikov, Ukraine’s defense minister. “They understand that engineering equipment right now is solving a key problem and is a game changer, so they want to destroy all of that first. I sent another letter to all of our partners to focus on this right now.”

The Washington Post’s Anastacia Galouchka contributed to this report.