Putin claims Russian lines are holding.

The Ukrainian military said on Thursday that its forces were pushing in two directions in southern Ukraine, aiming to rip through Moscow’s heavily fortified defensive lines, as President Vladimir V. Putin of Russia acknowledged a major uptick in attacks but claimed Russian lines were holding.

“We confirm that hostilities have intensified and in a significant way,” Mr. Putin said at a summit of African leaders in St. Petersburg, Russia.

It was the first time the Russian president had made direct comments about what U.S. officials have described as the start of Ukraine’s main thrust in its counteroffensive in the Zaporizhzhia region, involving thousands of soldiers newly outfitted with Western arms.

In the past, Mr. Putin rarely talked about the nitty-gritty details of military maneuvers on the battlefield, but in recent weeks he has sought to portray himself as well-informed about real-time events during Ukraine’s push to recapture territory.

There were few clear signs of progress in a Ukrainian campaign that military analysts have said will be long and difficult. The Ukrainian military, which has sought to maintain operational secrecy, gave no confirmation that its counteroffensive had entered a new phase.

Earlier on Thursday, Russia launched blistering artillery and aerial bombardments across southern Ukraine, the Ukrainian military said, an apparent effort to repel the intensifying assault. Russian forces are focusing their “main efforts on preventing the further advance of Ukrainian troops,” the Ukrainian military’s general staff said.

It said that Ukrainian forces “continue to conduct an offensive operation in Melitopol and Berdiansk directions,” two cities along the Sea of Azov, which Ukraine hopes to reach to drive a wedge through Russian-occupied territory in the south and east.

Privately, Ukrainian officials said that reserve units trained for the counteroffensive were not yet in use.

Two American officials, who spoke on the condition of anonymity to discuss details of the campaign, said they believed that Ukraine’s “main thrust” was taking place in the direction of Robotyne, which is about six miles from the Ukrainian-controlled town of Orikhiv and about 50 miles north of Melitopol.

The Russian Defense Ministry said on Thursday that Ukraine’s assault in the area had failed, suffering “heavy losses,” and that Kyiv’s forces did not go on the attack overnight.

The United States’ defense secretary, Lloyd J. Austin III, speaking to reporters while visiting Papua New Guinea on Thursday, did not comment on the claims that the counteroffensive was in a new phase. He said that Kyiv had done a good job “preserving manpower and equipment” despite months of bloody fighting, and that it still had “a number of options available to them.”

Mick Ryan, a retired Australian army major general, cautioned against making a snap assessment of the fighting, in part because Ukrainian forces were a long way from being able to penetrate Russia’s main defensive lines.

“These are very initial reports and first reports are almost always wrong,” he said.

Reaching Russian-occupied Melitopol is a key objective of the Ukrainian counteroffensive. Ukraine is also trying to advance eastward in the direction of the city of Bakhmut and south toward Berdiansk, a port city about 70 miles east of Melitopol, a direction in which it reported progress on Thursday.

Ukrainian Special Forces released video, verified by The New York Times, that appears to show Ukrainian soldiers taking Russian soldiers prisoner in the village of Staromaiorske. The village, about 90 miles north of Berdiansk, is near a cluster of farming hamlets along a winding river that Ukrainian forces claimed in the early days of their counteroffensive. It was not clear from the footage how much control Ukraine has established there.

Carlotta Gall contributed reporting from the Zaporizhzhia region of Ukraine, Haley Willis from Seoul and Matthew Mpoke Bigg from London.