In the first two weeks of Ukraine’s grueling counteroffensive, as much as 20 percent of the weaponry it sent to the battlefield was damaged or destroyed, according to American and European officials. The toll includes some of the formidable Western fighting machines — tanks and armored personnel carriers — the Ukrainians were counting on to beat back the Russians.

The startling rate of losses dropped to about 10 percent in the ensuing weeks, the officials said, preserving more of the troops and machines needed for the major offensive push that the Ukrainians say is still to come.

Some of the improvement came because Ukraine changed tactics, focusing more on wearing down the Russian forces with artillery and long-range missiles than charging into enemy minefields and fire.

But that good news obscures some grim realities. The losses have also slowed because the counteroffensive itself has slowed — and even halted in places — as Ukrainian soldiers struggle against Russia’s formidable defenses. And despite the losses, the Ukrainians have so far taken just five of the 60 miles they hope to cover to reach the sea in the south and split the Russian forces in two.

One Ukrainian soldier said in an interview this week that his unit’s drone picked up footage of a half-dozen Western armored vehicles caught in an artillery barrage south of the town of Velyka Novosilka.

“They all burned,” said the soldier, who identified himself as Sgt. Igor. “Everybody is hoping for a big breakthrough,” he said, adding a plea that those scrutinizing from afar appreciate the importance of slow and steady advances.

Russia had many months to prepare for the counteroffensive, and the front is littered with mines, tank traps and dug-in troops, while Russian reconnaissance drones and attack helicopters fly overhead with increasing frequency.

Given those fortifications, experts say, it is not surprising that Ukraine would sustain relatively severe losses in the early stages of the campaign.

This week, Ukraine’s president, Volodymyr Zelensky, acknowledged that there had been a brief pause in operations some weeks ago but blamed it on a lack of equipment and munitions, and called on Western allies to quicken the pace of deliveries.

American officials acknowledged that pause and said that the Ukrainians had begun moving again, but more deliberately, more adept at navigating minefields and mindful of the casualty risks. With the influx of cluster munitions from the United States, they said, the pace might pick up.

“It’s not as fast, but it’s not catastrophically behind schedule,” the British defense minister, Ben Wallace, said on Wednesday. “It is doing what anyone else would do having to fight through minefields towards the Russian line.”

The problems come into focus out in the farm fields in southern Ukraine where much of the counteroffensive is being fought. There the Bradley Fighting Vehicles, long coveted by the Ukrainians, have been running over anti-tank mines on a daily basis, soldiers who have fought in the vehicles say.

The vehicles, which weigh about 34 tons, are designed to carry infantry soldiers through areas exposed to gunfire or artillery. A rear ramp opens to allow soldiers to pile out and fight. In planning for the counteroffensive, the Bradleys were meant to carry soldiers across open fields to reach Russian trenches and bunkers.

The Bradleys have done part of their job well; their thick armor has provided good protection for most soldiers, who have survived many of the mine blasts with few injuries.

“Your ears ring and things inside fly around,” said one soldier, who asked to be identified only by his first name and rank, Pvt. Serhiy. He survived such an explosion last month in fighting south of the town of Orikhiv in the Zaporizhzhia region. But in many cases the blasts severely damaged the vehicles, immobilizing them well before they could reach the Russian lines.

Military experts have long said that the first 15 miles of the counteroffensive would be the hardest, as attacking troops generally need three times more power — whether in weapons, personnel or both — than defending forces.

Ukraine’s top military officer, Gen. Valery Zaluzhny, expressed frustration that Ukraine is fighting without Western F-16 warplanes, which the United States only recently agreed to allow Ukrainian pilots to be trained on, but which are not expected to be delivered for several months at least. That has left the Ukrainian troops vulnerable to the Russian helicopters and artillery.

Military analysts cautioned that it was still too early to draw definitive conclusions about the counteroffensive. “It does not mean that it is doomed to fail,” said Camille Grand, a defense expert at the European Council on Foreign Relations and a former NATO assistant secretary general.

Nevertheless, he added, the absence of air superiority and air defenses that Western jets could provide for Ukraine’s attack means “that casualty rates are likely to be higher than in other conventional conflicts.”

The precise numbers of weapons and armored vehicles that have been destroyed in the counteroffensive, as opposed to “mobility kills” that can be repaired, are closely guarded secrets, and the U.S. officials did not give raw numbers, though they did agree on the percentages of weaponry lost. But a combination of open source data and official estimates can provide a snapshot in time of the destruction, particularly in the early going.

Ukraine’s 47th Mechanized Brigade, one of the three Western-equipped and trained units that were deployed early in the campaign, was set to receive 99 Bradleys, according to the leaked U.S. military plans for the counteroffensive from February — still the most recent that have been made public.

Data from Oryx, a military analysis site that counts only losses that it has visually confirmed, show that 28 of those Bradleys have been abandoned, damaged or destroyed, including 15 in a village in Zaporizhzhia Province on June 8 and 9 as the 47th was attacked by helicopters while trapped in a minefield. Six additional Bradleys were reported abandoned or destroyed in Mala Tokmachka on June 26, but Oryx researchers said these losses had occurred earlier, although it is not clear exactly when.

Given that the 47th was the only brigade initially slated to receive the Bradleys, that means that nearly one-third of the original vehicles have been lost — although all but seven of them were blown up at one battleground.

“It is within the realm of possibility that Ukrainian forces have seen losses at this level,” said Dylan Lee Lehrke, an analyst with the British security intelligence firm Janes, adding that a “significant” level of lost weapons was generally a hallmark of wars of attrition, like the one in Ukraine.

The Oryx data show that only 24 tanks were lost for the entire month of June, including some from Ukraine’s own arsenal in addition to those supplied by Western allies.

Ten of them were German-made Leopard tanks and mine-clearers, the data show. Presumably, they were lost in battle with Ukraine’s 33rd Mechanized Brigade, one of the three units deployed early in the counteroffensive, and which was slated to receive 32 Leopards in the U.S. planning documents from Feb. 28.

That would mean that the brigade lost 30 percent of the Leopards it was given — all but two of them in the first week of fighting, the Oryx data show.

The Ukrainian authorities say the army has so far advanced the deepest in southern areas of the Donetsk region, but no more than about five miles from the former front line at Velyka Novosilka. It faces another 55 miles to reach the Sea of Azov, a primary goal of the counteroffensive, as it would cut the land bridge to Crimea, wreaking havoc with Russia’s already shaky logistics. Ukraine’s forces are also advancing in two areas in the Zaporizhzhia region.

It is even slower near Orikhiv in the Zaporizhzhia region, where the bulk of Bradleys and Leopards have been sent to an area of open fields with little cover, There, Ukraine’s army has advanced only about a mile.

Justin Scheck contributed reporting from London.