Military analysts and U.S. officials said it was too soon to judge the success of Ukraine’s offensive, which is looking for weaknesses to exploit, in the face of fierce resistance.
Intense fighting raged across a wide swath of southeastern Ukraine for a second day on Friday, as Ukrainian forces attacked occupying Russian troops in multiple locations, while military analysts and U.S. officials cautioned that it was far too early to gauge the success of Kyiv’s offensive.
Both sides were grappling with severe flooding caused by the destruction of a major dam on the Dnipro River, but east of there, the fierce combat indicated that Ukraine’s long-awaited counteroffensive against the Russian invasion was underway, according to analysts and Western and Russian officials.
Two senior U.S. officials, speaking on the condition of anonymity to discuss sensitive military operations, confirmed that Ukrainian troops had, as expected, suffered casualties and equipment losses in the early fighting, but said that classified assessments quantifying the losses were still being developed.
There was no information available on Russian losses, but attackers typically suffer heavier initial casualties than dug-in defenders, and analysts warned that breaking through the Russian lines would be difficult and come at a high price.
The Russians have constructed layers of formidable defenses — with trenches, bunkers, minefields, concrete tank obstacles and gun emplacements — and the flat ground leaves advancing troops vulnerable to Moscow’s artillery and air power.
Videos and photos posted by pro-war Russian bloggers, and verified by The New York Times, show that at least three German-made Leopard 2 tanks and eight American-made Bradley fighting vehicles were recently abandoned by Ukrainian troops or destroyed.
The Pentagon on Friday announced another round of military aid to Ukraine, this time worth $2.1 billion, including air defense missiles and artillery shells.
Both warring countries gave positive-sounding but vague assessments that offered minimal detail about the battle. President Volodymyr Zelensky of Ukraine said in a video address on Thursday night that his forces were achieving “step by step” results, but did not say what those results were, and the Ukrainian military said on Friday that the “enemy remains on the defensive.”
President Vladimir V. Putin of Russia said on Friday, at a public appearance in Sochi, Russia, that Ukraine’s counteroffensive had begun, as evidenced by its use of “strategic reserves.” Ukraine’s military had not made progress, he said, but still had “offensive potential.”
Pentagon officials and military analysts have been increasingly bullish on Ukraine’s prospects for taking back much of the 18 percent of the country that Russia still occupies.
“This isn’t something you judge based on a few days of fighting,” Michael Kofman, the director of Russian studies at CNA, a research institute in Arlington, Va., said in a Twitter message on Friday. “The offensive will play out over weeks and likely months.”
The Institute for the Study of War, an analysis group based in Washington, wrote on Thursday evening that Ukraine had not yet committed all of the newly trained and equipped units it had prepared for the offensive, and was fighting in the Zaporizhzhia region against Russian units that were considerably stronger than those elsewhere along the front.
The Ukrainians have attacked in several places in Zaporizhzhia and the adjacent Donetsk region, looking for weaknesses to exploit, and are expected to shift troops and equipment to concentrate on those vulnerabilities.
Some of the heaviest fighting has been reported, by officials on both sides, near the town of Orikhiv, in the southern Zaporizhzhia region. Analysts have long anticipated a major Ukrainian thrust there, pushing southward toward the city of Melitopol and the Sea of Azov, in an attempt to cut in two the land that Russia has seized.
Zaporizhzhia is where Moscow has “designed one of the largest defensive systems in Europe since World War II,” said analysts with the Center for Strategic and International Studies, a Washington-based research group, in a report released on Friday. The report, based on satellite data, says defenses there are more than six miles deep, more than double those erected in other regions.
Beginning on Thursday, Russian pro-war bloggers and the Russian military reported that Ukraine had unsuccessfully attempted an advance a few miles east of Orikhiv, near the village of Mala Tokmachka.
Images posted by a Ukrainian brigade and verified by The Times show that Ukrainian troops were on foot in Lobkove, a settlement west of Orikhiv.
Ukraine was also attacking to the northeast, in the Donetsk region, around the town of Velyka Novosilka and the city of Bakhmut, which fell last month to Russian forces after the longest and bloodiest battle of the war. Ukrainian forces reporting gaining ground on the city’s flanks.
This Ukrainian counteroffensive is expected to be one of the largest military operations in Europe since World War II, involving tens of thousands of soldiers and hundreds of tanks, armored vehicles and howitzers, fighting in and around farm fields, towns and villages.
Each side has run low on munitions at times, with Ukraine relying on its Western backers, and Russia buying attack drones from Iran. The Biden administration on Friday released newly declassified intelligence on a drone factory Russia is building with Iranian help, saying that it could be in operation by next year.
Farther west, in the Kherson region, the two sides are separated by the Dnipro River, now much wider and surrounded by devastation after the destruction on Tuesday of the Kakhovka dam. The would make a Ukrainian advance across the river much more difficult, but Ukrainian officials say such an assault was not in their plans, and the dam breach will have no effect on the course of the fighting.
A senior Biden administration official said that U.S. spy satellites detected an explosion at the Kakhovka dam just before it collapsed, but American analysts still do not know what — or who — caused it.
Separately, a Norwegian seismic monitoring foundation reported that its equipment in Romania had detected two explosions from the direction of the dam. There was a weaker one at 2:35 a.m. on Tuesday, and then a stronger one, with movement equivalent to a magnitude 1 to 2 earthquake, at 2:54 a.m., about the time that the dam broke, the group said.
Experts say the dam, which was held by Russian forces, was probably destroyed by an intentional explosion within the massive structure. They say an explosion from the outside, like a missile strike, or a structural failure caused by earlier war damage and high water spilling over the top, were conceivable causes but far less likely.
Ukraine’s government says the only plausible scenario is that the Russians, who were in control of the dam, blew it up. Its security service on Friday released an audio clip of what it said was an intercepted phone call, in which a man it identified as a Russian soldier said “it was our sabotage group” who destroyed it, and added that the damage was “more than they planned.” The recording’s validity could not be determined.
Russian officials have blamed Ukraine, offering varied scenarios but no evidence. Some commentators on Russian state television have celebrated the dam’s destruction.
The flooding has forced thousands of people from their homes on both banks of the Dnipro, washed away entire buildings and clogged the river with debris and toxins. On the Russian-held side, officials said eight people were killed. On the Ukrainian side, five were reported dead and 13 missing.
Ukrainian officials said on Friday that Russian forces had shelled areas where evacuations and rescues were underway, killing two people.
Flood waters were receding in the city of Kherson but not downstream, near where the river empties into the Black Sea. Debris from the flood littered faraway seashores; residents of Odesa, more than 70 miles from the mouth of the Dnipro, reported seeing roofs of houses and dead animals floating nearby.
Aid groups and Ukrainian officials warned that the flood had washed away many land mines, sending them tumbling toward the sea, and that other debris could collide with and detonate floating naval mines. The land mines, some of which have detonated, pose a lethal risk to people on or near the waters.
Natalia Humeniuk, the spokeswoman for the Ukrainian military southern command, said that even seemingly innocuous-looking materials washing up on shores as far as Odesa could contain explosive devices.
Marc Santora, Paul Sonne, Christiaan Triebert, Haley Willis, Helene Cooper, Julian E. Barnes, Christoph Koettl and Gabriela Sá Pessoa contributed reporting.