NYT article The officials spoke on condition of anonymity to discuss details of the campaign. Their comments dovetailed with reports from the battlefield on Wednesday, where artillery battles flared along the southern front line in the Zaporizhzhia region.

And Igor Konashenkov, the Russian Defense Ministry's chief spokesman, reported a "massive" assault and fierce battles south of Orikhiv, a town that Ukraine holds about 60 miles north of the Sea of Azov. Vladimir Rogov, an official appointed by Moscow in southern Ukraine, said the assault involved Ukrainian troops who had been trained abroad and were equipped with about 100 armored vehicles, including German-made Leopards and American-made Bradley Fighting Vehicles.

Another Russian occupation official in Zaporizhzhia, Yevgeny Balitsky, said that Ukraine had made 36 attempts to shell settlements in the region since Tuesday. Russian assertions that the Ukrainian attacks had been repelled could not be immediately verified.

Ukrainian troops along the southern front said in interviews on Wednesday that they were steadily pushing Russian troops back, but their progress had been incremental with no major breakthroughs. They have been slowed by minefields, and some said the biggest obstacles were Russia's withering artillery fire and airstrikes.

Ukrainian officials have told U.S. officials that the enlarged Ukrainian force would try to advance south through Russia's minefields and other fortifications toward the city of Tokmak, and, if successful, on to Melitopol, near the coast.

Their goal is to sever the so-called land bridge between Russian-occupied Ukraine and the Crimean peninsula, or at least advance far enough to put the strategically important peninsula within range of Ukrainian artillery. Russia seized Crimea from Ukraine in 2014, and uses it as the base for its Black Sea fleet as well as to supply its forces in the south.

The new operation, if successful, could take one to three weeks, Ukrainian officials have told officials in Washington.

However, little has gone according to plan since the counteroffensive started early in June, and officials at the White House and Pentagon said on Wednesday they were watching the increased activity with keen interest.

"This is the big test," said one senior official, who spoke on condition of anonymity to discuss internal deliberations.

For more than seven weeks, Ukrainian troops have fought along three main fronts across several hundred miles in the country's east and southeast, pressing to find a weak spot in the heavily dug-in Russian defenses to burst through.

American officials said on Wednesday that Ukrainian officials told them that Ukraine was now engaged in the main thrust of the counteroffensive, throwing more troops and equipment at the westernmost of the three fronts, near Zaporizhzhia.

The officials, speaking on condition of anonymity to discuss Ukraine's war plans, cited three reasons.

First, Ukrainian forces have been making plodding but steady progress clearing a path through the dense Russian minefields and other fortifications. Second, they sensed an opportunity with the sacking of the regional Russian commander, Maj. Gen. Ivan Popov, to exploit turmoil in the local Russian leadership.

Last month, General Popov addressed his troops in a four-minute recording, accusing his superiors of inflicting a blow on his forces by removing him from his post in retaliation for voicing the truth about battlefield problems to senior leadership behind closed doors.

And third, Ukrainian artillery barrages have been steadily attacking Russian artillery, ammunition depots and command posts in areas well behind the front lines, creating a vulnerability to exploit if advancing Ukrainian forces can punch through the Russian defenses, and cause havoc in the rear.

"The Russians are stretched," a Western official said on Wednesday, speaking on condition of anonymity to discuss operational details and intelligence assessments. "They are still experiencing problems with logistics, supply, personnel and weapons. They're feeling the pressure."

The sound of artillery battles could be heard in villages all along the front line in southern Ukraine on Wednesday. Ukrainian guns fired steadily from hidden positions and Russian artillery and mortars targeted former Russian positions and villages now occupied by Ukrainian soldiers.

Ukrainian troops deployed along the southern front as part of the counteroffensive say they are steadily pushing the Russian troops back, but they describe their progress as step by step, rather than breakthrough movements.

The front in Zaporizhzhia is one of three axes of Ukraine's counteroffensive. One Ukrainian military objective is to reach the Sea of Azov, driving a wedge between Russian-occupied territories in southern and eastern Ukraine.

But Russian forces have built a layered defense of minefields, tank traps and fortified positions that stretches along the length of the front line and runs at least 20 miles deep. In the two months since their counteroffensive began, the Ukrainians have had to cross these heavily mined fields and tree lines to take dug-in Russian positions, while coming under heavy bombardment and frequent airstrikes.

The devastating artillery fire and powerful aviation bombs have hampered them the most, soldiers on the front line said.

On Wednesday morning a Ukrainian jet flew low over the front line, firing rockets at Russian positions before banking away sharply and releasing flares to deflect any antiaircraft fire.

Some of Ukraine's newly trained brigades, many of them equipped with Western armored fighting vehicles and personnel carriers, are engaged in the fighting alongside well-established units of marines and mechanized infantry. The Ukrainian army command has said that not all the new Western-provided weapons are in use yet.

The strength of Russia's firepower has forced the Ukrainians to adjust their tactics, but Ukrainian officials say they are still progressing and in some places have succeeded in breaching the first line of Russian defenses.

That may open the way to faster progress in coming weeks, some officials have hinted, but the army command is also focused on straightening the front line to prevent bulges that would expose the advancing units to a flanking movement.

Ukrainian forces launched a significant attack south of the town of Orikhiv in the Zaporizhzhia region, Russia's Defense Ministry said on Wednesday, suggesting that a new phase of Kyiv's southern counteroffensive might be underway.

Ukraine deployed three battalions reinforced with tanks, said Igor Konashenkov, the Russian Defense Ministry's chief spokesman, but he added that the "massive" attack and a second one near Robotyne, a Russian-held frontline village around eight miles southeast of Orikhiv, had been repelled.

Mr. Konashenkov's comments, reported by the Russian state news agency Tass, could not be independently confirmed, and Ukrainian officials did not directly comment. But the area south of Orikhiv, a town that Ukraine holds, is a key battleground in its push to expel Russian forces from the south and east of the country, and both sides have built up their forces in the area.

Ukraine's counteroffensive — launched last month along three axes including in Zaporizhzhia — has failed to score a breakthrough, in part because of dense Russian minefields and heavy entrenchment by Moscow's forces.

A major Ukrainian military objective is to reach the Sea of Azov, about 60 miles south of Orikhiv, in order to drive a wedge through Russian-occupied territory in Ukraine's southeast.

Vladimir Rogov, an official appointed by Moscow in southern Ukraine, said on the Telegram app that fierce battles had begun south of Orikhiv, involving Ukrainian troops who had been trained abroad and were equipped with about 100 armored vehicles, including German-made Leopard tanks and American-made Bradley Fighting Vehicles.

Another Russian occupation official in Zaporizhzhia, Yevgeny Balitsky, said that Ukraine had made 36 attempts to shell settlements in the region since Tuesday. He said that Moscow's forces had rebuffed Ukrainian attempts to break through Russian defenses.

The Ukrainian military's general staff said in its daily update on Wednesday that Russian forces were engaged in defensive operations in the Zaporizhzhia region. A spokesman for the general staff, Andriy Kovalev, said that Russian forces had unsuccessfully tried to restore lost positions northeast of Robotyne. "The enemy continues to put up strong resistance, moves units and actively uses reserves," he said.

President Volodymyr Zelensky of Ukraine said that he had met with Gen. Valery Zaluzhny, the commander of Ukraine's armed forces, and discussed offensive and defensive fighting on the front line. "We believe in our boys," Mr. Zelensky said on Telegram, giving no details of military operations. "We continue to work."

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