The first several weeks of Ukraine’s long-awaited counteroffensive have not been kind to the Ukrainian troops who were trained and armed by the United States and its allies.

Equipped with advanced American weapons and heralded as the vanguard of a major assault, the troops became bogged down in dense Russian minefields under constant fire from artillery and helicopter gunships. Units got lost. One unit delayed a nighttime attack until dawn, losing its advantage. Another fared so badly that commanders yanked it off the battlefield altogether.

Now the Western-trained Ukrainian brigades are trying to turn things around, U.S. officials and independent analysts say. Ukrainian military commanders have changed tactics, focusing on wearing down the Russian forces with artillery and long-range missiles instead of plunging into minefields under fire. A troop surge is underway in the country’s south, with a second wave of Western-trained forces launching mostly small-scale attacks to punch through Russian lines.

But early results have been mixed. While Ukrainian troops have retaken a few villages, they have yet to make the kinds of sweeping gains that characterized their successes in the strategically important cities of Kherson and Kharkiv last fall. The complicated training in Western maneuvers has given the Ukrainians scant solace in the face of barrage after barrage of Russian artillery.

Ukraine’s decision to change tactics is a clear signal that NATO’s hopes for large advances made by Ukrainian formations armed with new weapons, new training and an injection of artillery ammunition have failed to materialize, at least for now.

It raises questions about the quality of the training the Ukrainians received from the West and about whether tens of billions of dollars’ worth of weapons, including nearly $44 billion worth from the Biden administration, have been successful in transforming the Ukrainian military into a NATO-standard fighting force.

“The counteroffensive itself hasn’t failed; it will drag on for several months into the fall,” said Michael Kofman, a senior fellow at the Carnegie Endowment for International Peace who recently visited the front lines. “Arguably, the problem was in the assumption that with a few months of training, Ukrainian units could be converted into fighting more the way American forces might fight, leading the assault against a well-prepared Russian defense, rather than helping Ukrainians fight more the best way they know how.”

President Vladimir V. Putin of Russia has increasingly signaled that his strategy is to wait out Ukraine and its allies and win the war by exhausting them. American officials are worried that Ukraine’s return to its old tactics risks that it will race through precious ammunition supplies, which could play into Mr. Putin’s hands and disadvantage Ukraine in a war of attrition.

Biden administration officials had hoped the nine Western-trained brigades, some 36,000 troops, would show that the American way of warfare was superior to the Russian approach. While the Russians have a rigidly centralized command structure, the Americans taught the Ukrainians to empower senior enlisted soldiers to make quick decisions on the battlefield and to deploy combined arms tactics — synchronized attacks by infantry, armor and artillery forces.

Western officials championed that approach as more efficient than the costly strategy of wearing Russian forces down by attrition, which threatens to deplete Ukraine’s ammunition stocks.

Much of the training involved teaching Ukrainian troops how to go on the offensive rather than stay on defense. For years, Ukrainian troops had worked on defensive tactics as Russian-backed separatists launched attacks in eastern Ukraine. When Moscow began its full-scale invasion last year, Ukrainian troops put their defensive operations into play, denying Russia the swift victory it had anticipated.

The effort to take back their own territory “is requiring them to fight in different ways,” Colin H. Kahl, who recently stepped down as the Pentagon’s top policy official, said last month.

But the Western-trained brigades received only four to six weeks of combined arms training, and units made several mistakes at the start of the counteroffensive in early June that set them back, according to U.S. officials and analysts who recently visited the front lines and spoke to Ukrainian troops and commanders.

Some units failed to follow cleared paths and ran into mines. When a unit delayed a nighttime attack, an accompanying artillery bombardment to cover its advance went ahead as scheduled, tipping off the Russians.

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

Military experts said that using newly learned tactics for the first time was always going to be hard, especially given that the Russian response was to assume a defensive crouch and fire massive barrages of artillery.

“They were given a tall order,” said Rob Lee, a Russian military specialist at the Foreign Policy Research Institute in Philadelphia and a former U.S. Marine officer, who has also traveled to the front lines. “They had a short amount of time to train on new equipment and to develop unit cohesion, and then they were thrown into one of the most difficult combat situations. They were put in an incredibly tough position.”

President Volodymyr Zelensky of Ukraine acknowledged in late July that his country’s counteroffensive against dug-in Russian troops was advancing more slowly than expected.

“We did have plans to start it in the spring, but we didn’t because, frankly, we had not enough munitions and armaments and not enough properly trained brigades — I mean, properly trained in these weapons,” Mr. Zelensky said via video link at the Aspen Security Forum, an annual national-security conference.

He added that “because we started it a bit late,” Russia had “time to mine all of our lands and build several lines of defense.”

Ukraine may well return to the American way of warfare if it breaks through dug-in Russian defenses, some military experts said. But offense is harder than defense, as Russia demonstrated last year when it abandoned its initial plans to advance to Kyiv.

“I do not think they’re abandoning combined arms tactics,” Philip M. Breedlove, a retired four-star Air Force general who was NATO’s supreme allied commander for Europe, said in an interview. “If they were to get through the first, second or third lines of defense, I think you’re going to see the definition of combined arms.”

Speaking at the Aspen forum, Jake Sullivan, President Biden’s national security adviser, said, “Ukraine has a substantial amount of combat power that it has not yet committed to the fight, and it is trying to choose its moment to commit that combat power to the fight when it will have the maximum impact on the battlefield.”

That moment appeared to come last week when Ukraine significantly ratcheted up its counteroffensive with two southward thrusts apparently aimed at cities in the Zaporizhzhia region: Melitopol, near the Sea of Azov, and Berdiansk, to the east on the Azov coast. In both cases, the Ukrainians have advanced only a few miles and have dozens more to go.

But analysts question whether this second wave, relying on attacks by smaller units, will generate enough combat power and momentum to allow Ukrainian troops to push through Russian defenses.

Gian Luca Capovin and Alexander Stronell, analysts with the British security intelligence firm Janes, said that the small-unit attack strategy “is extremely likely to result in mass casualties, equipment loss and minimal territorial gains” for Ukraine.

U.S. officials said, however, the surge in Ukrainian forces in the past week came at a time when the Ukrainians were clearing paths through some of the Russian defenses and beginning to wear down Russian troops and artillery.

A Western official, speaking on the condition of anonymity to discuss operational details and intelligence assessments, said the Russians were stretched and still experiencing problems with logistics, supply, personnel and weapons.

General Breedlove concurred and said he still expected the Ukrainian counteroffensive to put Russia at a disadvantage.

“The Ukrainians are in a place now where they understand how they want to employ their forces,” he said. “And we’re starting to see the Russians move backwards.”

Thomas Gibbons-Neff contributed reporting from London.