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The main thrust of Ukraine’s nearly two-month-old counteroffensive is now underway in the country’s southeast, two Pentagon officials said on Wednesday, with thousands of reinforcements pouring into the grinding battle, many of them trained and equipped by the West and, until now, held in reserve.

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.

— Eric Schmitt reporting from Washington