KYIV, Ukraine — The Ukrainian military has launched a long-anticipated counteroffensive against occupying Russian forces, opening a crucial phase in the war aimed at restoring Ukraine’s territorial sovereignty and preserving Western support in its fight against domination by Moscow.

Ukrainian troops, including specialized attack units armed with Western weapons and trained in NATO tactics, intensified their strikes on front-line positions in the country’s southeast Wednesday night, according to four members of the country’s armed forces, beginning a significant push into Russian-occupied territory.

The four military personnel, including officers, spoke on the condition of anonymity because they were not authorized to publicly discuss the battlefield developments. Ukrainian officials have said repeatedly in recent days that they would not make an official announcement that the counteroffensive had begun, and that no single action would mark its start.

Russian Defense Minister Sergei Shoigu said Ukrainian forces tried to break through the lines of the Russian army in the Zaporizhzhia region, using up to 1,500 troops and 150 armored vehicles. His claims could not be independently verified.

The Zaporizhzhia region has long been seen as the most strategic and likely location of the new Ukrainian campaign, which is expected to unfold over the course of months. The offensive will be a pivotal test of the U.S.-led strategy to prepare Ukrainian forces with more advanced tactics and weaponry. It will also test the resolve of Russia’s beleaguered forces, which are beset by infighting and have already endured numerous high-profile setbacks in the 15 months since President Vladimir Putin ordered the full-scale invasion of Ukraine.

By cutting south through the Zaporizhzhia region’s flat fields, Kyiv’s forces could aim to sever the “land bridge” between mainland Russia and occupied Crimea, cutting off crucial Russian supply lines. They could also attempt to liberate the city of Melitopol, which Russia has established as the region’s occupied capital, and Enerhodar, where the Zaporizhzhia nuclear power plant is located.

But fierce obstacles stand in Ukraine’s way. Russian forces have spent months fortifying the area with mines and trenches. A member of one brigade taking part in the offensive in the southeast described “continuous heavy fighting.”

“It is very difficult on the battlefield,” he said Thursday morning. “Our artillery and aviation are working, but the Russians’ are working as well. It is difficult for us and for them. The armed forces are advancing. But not as fast as we wanted.”

Video released by the Russian Ministry of Defense on Thursday and independently verified by The Washington Post showed a Ukrainian tank being destroyed in the Zaporizhzhia region. An aerial photo of the same scene showed a column of at least six Ukrainian military vehicles, which appear to include German-made Leopard 2 tanks, according to two military analysts.

The column came under fire from artillery shells, said Mark Cancian, a retired U.S. Marine Corps colonel and senior adviser at the Center for Strategic and International Studies in Washington. While artillery shells typically do not destroy tanks, Cancian added, this shell appeared to land directly on the armored vehicle.

Alexander Romanchuk, the Russian commander in the area, told journalists Thursday that Ukrainian forces had lost more than 30 tanks, including three Leopards, and were forced to retreat. His claims could not be independently verified.

Valeriy Shershen, a spokesman for Ukrainian military units located across much of the eastern and southern front line, confirmed “more activity” in the Zaporizhzhia region but added that he “wouldn’t say it’s something major.” In the Zaporizhzhia region, Ukrainian forces are generally “still in a defense operation,” Shershen said in an interview.

Shershen also described “small counteroffensive activities” of a local scale, particularly the fight for Velyka Novosilka, a village in the Donetsk region just east of Zaporizhzhia. The Russians, he added, have stepped up their shelling in the Zaporizhzhia region in anticipation of a possible Ukrainian attack.

Ukrainian Deputy Defense Minister Hanna Maliar wrote on Telegram Thursday that battles were underway for Velyka Novosilka. Maliar also said the Russians were on the “active defense” near Orikhiv, a town close to the front in Zaporizhzhia.

Yevgeniy Prigozhin, head of Russia’s Wagner mercenary group, said in a Telegram statement Thursday that “this first two-day wave is the beginning of a counteroffensive” and “certainly it will continue.”

The counteroffensive is intensifying as a crisis builds in Ukraine’s southern Kherson region, where a breach of the Russian-controlled Kakhovka dam and hydroelectric power plant Tuesday sent water rushing over the banks of the Dnieper River and into dozens of residential communities in Ukrainian- and Russian-controlled territories.

Ukrainian authorities and volunteers evacuated Kherson residents Thursday amid shelling from Russian forces on the other side of the river. At least one woman was killed and 17 people were injured by incoming fire Thursday, according to Oleksandr Prokudin, Ukraine’s governor of Kherson. The injured included two workers from Ukraine’s state emergency services.

Serhiy Kruk, head of Ukraine’s state emergency services, reported “massive artillery shelling of locations where our rescuers work” as they tried to evacuate civilians to a “seemingly safe territory.”

Earlier Thursday, Ukrainian President Volodymyr Zelensky visited inundated areas of Kherson, speaking to evacuees and local officials. He urged international humanitarian agencies to ramp up their aid response and called the situation “absolutely catastrophic.”

Putin has no plans to visit flood-hit areas of Ukraine’s Kherson region, parts of which are controlled by Russian forces, the Kremlin said Thursday. Last fall, Putin declared his intention to annex the Kherson region, only to see his troops pushed to retreat and surrender territory, including the regional capital.

The cause of the dam collapse, unleashing one of the largest reservoirs in Ukraine, remains unclear. Zelensky has accused Russia of a deliberate attack causing an explosion inside the hydroelectric power plant. Russia, which seized the dam at the start of its invasion last year, has accused Ukraine of destroying it to cut off water to occupied Crimea. But authorities in Moscow have not explained how Ukraine could have done so with the plant under Russian control.

Ukraine’s state hydroelectric company said Thursday that water levels in the Kakhovka reservoir continue to drop. More than 2,198 people have been evacuated, including 120 children, Prokudin said in a Telegram video Thursday; the booms of Russian shelling could be heard in the background as he spoke. More than two dozen communities and 3,400 homes on the western, Ukrainian-controlled bank of the Dnieper River have been flooded.

The massive flooding has redrawn the battlefield in that part of the southern front. But Natalia Humeniuk, a spokeswoman for the Ukrainian military’s southern command, said troops have not been affected, as they knew how the water from the dam’s reservoir would flow. The west bank is higher in elevation than the east, she said, so the damage to Ukraine’s military has been “minimal.”

Konrad Muzyka, president of Rochan Consulting, a military analysis firm based in Poland, said the Ukrainian offensive has been building for several weeks.

The Ukrainians, he added Thursday morning, are “getting closer to the main phase.”