BRUSSELS — Senior Pentagon officials predicted on Thursday a long, violent battle as Ukrainian forces attempt to drive occupying Russian troops out of their country, offering their most candid comments to date about a new offensive campaign reliant on Western military equipment and training.

Gen. Mark A. Milley, chairman of the Joint Chiefs of Staff, said it would be "very premature" to estimate how long Ukraine's counteroffensive could take, considering that Russia has several hundred thousand troops dug in along the front lines. But he characterized the operation as "a very violent fight" that will "likely take a considerable amount of time - and at high cost."

Defense Secretary Lloyd Austin, appearing alongside Milley during a news conference at NATO headquarters, noted that the Kremlin had begun to circulate battlefield imagery of smoldering Western combat vehicles. Ukrainian forces, he said, have the ability to recover damaged military equipment and repair it.

"The Russians have shown us [the] same five vehicles about a thousand times from 10 different angles," Austin said, downplaying the significance of Ukraine's losses so far in the counteroffensive. "Quite frankly, the Ukrainians . . . still have a lot of combat capability." He added that whichever side is better able to sustain its combat forces "will probably have the advantage at the end of the day."

The Americans' remarks came as Ukrainian officials said that the country's military had retaken about 40 square miles of territory from Russian occupying forces and that Kyiv's counteroffensive was advancing "gradually but surely." Earlier this week, the Ukrainian military said it had liberated seven small villages in southeastern Ukraine, most clustered near the border of the Donetsk and Zaporizhzhia regions.

Russia's Defense Ministry has characterized Ukraine's attempts to retake lost territory as "unsuccessful," saying Kyiv's forces had suffered "significant losses in manpower and equipment." Moscow also claimed to have thwarted dozens of attacks and reported a successful strike on a warehouse storing ammunition and equipment - but it did not specify where.

Neither side's claims could be independently verified.

Thursday's news conference in Brussels concluded a meeting of defense officials from dozens of countries assisting Ukraine in the war. Collectively, they have provided tens of billions of dollars in military assistance, including an increasingly sophisticated array of weapons that now includes Western battle tanks and the prospect of F-16 fighter jets.

It did not appear that the Ukraine Defense Contact Group, as the coalition is known, had settled on a plan for outfitting Ukraine with F-16s. Milley praised the Netherlands and Denmark, which last month agreed to lead a training program for Ukrainian air and ground crews on how to fly and maintain the advanced aircraft, but he said that the effort "will take some time" and that there remains "a lot of work to do."

"You have to do language training. You have to do pilot training. You've got to get all the systems set in place," Milley said. "Those wheels are in motion, but we're a ways from completion of that project."

In separate remarks, Ukrainian Defense Minister Oleksii Reznikov spoke excitedly about "the so-called bird coalition" and said he had a commitment from several countries that training will occur. In addition to the Netherlands and Denmark, Reznikov said, Sweden has agreed to some form of "testing" in the effort. Sweden makes the Gripen fighter jet and last month signaled that, while Stockholm has no near-term plan to transfer any aircraft, it may let Ukraine learn how to operate them.

The meeting took place as Russia continues to bombard Ukrainian cities and civilian infrastructure with long-range missiles and one-way attack drones. Following another round of Russian strikes on cities across Ukraine, Hanna Maliar, Ukraine's deputy defense minister, said earlier this week that "extremely fierce battles" are raging along Ukraine's 600-mile front line, as Kyiv amplifies its counteroffensive.

NATO Secretary General Jens Stoltenberg has expressed cautious optimism at the incremental progress of Ukrainian forces, saying they are "making advances and liberating more land." But Stoltenberg, too, has conceded that it was too early to know whether the Ukrainian effort would be a "turning point" in the conflict and warned of brutal battles ahead.

"Nobody expected there to be zero casualties," he said. "There is fierce fighting going on."

A Western official, speaking on the condition of anonymity to brief the media, predicted "grinding, costly warfare for many months to come."

U.S. officials have been hesitant to address how the counteroffensive is progressing, but they are closely monitoring the fighting, with a senior U.S. military official saying efforts already are underway to incorporate early takeaways into the Western combat training being provided to Ukrainian forces.

"You don't want to fall into the logical fallacy of saying that the first time something happens, it's a trend," the senior U.S. military official said, appearing to reference imagery on social media intended to suggest Ukraine's efforts thus far have been ineffective.

Austin, asked about the adaptation of training over time, said Thursday that the ways in which squads and platoons of Ukrainian combat troops are observed fighting will be incorporated "into the training that we're doing for follow-on forces."

U.S. military officials said that more than 57,000 Ukrainian troops have been trained by Western allies since Russia's invasion in February 2022, including more than 11,000 by the United States. U.S. efforts have involved training 12 battalions of Ukrainian troops at two combat ranges in Germany, including three units on wheeled Stryker vehicles, three units on heavily armored Bradley infantry fighting vehicles, and six units on Humvees and other vehicles used by infantry soldiers.

The current U.S. training effort also includes instruction on M1 Abrams tanks, which the Biden administration pledged to Ukraine months ago as part of an arrangement that earlier this year included the transfer of German-made Leopard battle tanks from several European allies. The Abrams training began in May and is expected to be complete around the end of the summer.

In recent weeks, the United States and its allies have sought to steer conversations toward providing military support for Ukraine beyond the current offensive, with an eye to creating security arrangements that could deter Russia long-term. Diplomats from several member states are discussing bilateral or multilateral agreements with Ukraine along the lines of the agreements that channel billions of dollars in U.S. military aid to Israel.

The idea of turning Ukraine into a "porcupine" that could deter or repel future Russian attacks has strong support from allies but falls short of Kyiv's demands to join the alliance. Nothing other than membership would bring Ukraine under NATO's mutual defense pledge.

Stern reported from Kyiv. The Washington Post's Francesca Ebel in London contributed to this report.