After two months of painstakingly slow progress on the battlefield, Ukraine appears to be escalating its counteroffensive, ramping up the deployment of extra troops to the southern front and signaling a new phase of the operation, US and Russian officials said.

Ukraine has committed more forces to the southeast of the country, a sign that Kyiv has identified potential weaknesses in Russian defensive lines, two US officials told CNN.

The Ukrainian military had been holding large numbers of trained troops, some equipped with more powerful Western weapons, back since the operation started in early June. While it still maintains some combat power in reserve, it has now deployed the "main bulk" of the forces committed to the counteroffensive forces, one of the US officials said.

The thrust appears to have brought some results. The counteroffensive has broken through some elements of Russian defensive lines in the southeast, the US official said, and the reserve units have come in to capitalize on the opportunity.

A Russian military official admitted that Ukraine forces have been able "to wedge in" three sections of Russia's first line of defense in the Zaporizhzhia front line on Wednesday.

"The second wave of the [Ukrainian] counteroffensive has begun" on the Zaporizhzhia front, said Vladimir Rogov, a member of the Russian-installed military-civilian administration in the region. Ukrainian troops had managed to break in "as a result of several waves of attack with more than 100 units of armored vehicles," Rogov added, saying heavy fighting was ongoing along the southern front.

Ukraine's military did not comment in detail on the situation in the area beyond saying its offensive operations along the southern front were "gradually advancing."

Rogov went on to say Russian forces were using their full arsenal, including aviation strikes, to push back against the Ukrainian units carrying out the assault, which he claimed were Western-equipped and trained.

"The fighters of these brigades have been trained abroad, and the brigades themselves are equipped with Western military equipment, including Leopard tanks and Bradley [infantry fighting vehicles]," he wrote on Telegram.

Russian President Vladimir Putin said Thursday that "hostilities have intensified and in a significant way" on the front lines, but contradicted his officials on the ground and claimed Ukraine's push was being turned back.

"The enemy was not successful in any of the areas of clashes. All counteroffensive attempts were stopped, and the enemy was driven back with heavy losses," the Russian leader said.

Ukraine's 47th Brigade, which is involved in the offensive, confirmed to CNN it was pushing ahead, saying: "Infantry is advancing. Artillery covers. Bradley crews destroy enemy infantry and their equipment."

There has been little independent evidence of the state of the front lines, but Russian military bloggers and official Ukrainian accounts suggest that Ukrainian forces have cut a path through at least one of the many minefields laid by the Russians and made modest territorial gains in the area of Robotyne, south of the town of Orikhiv.

Ukraine's President Volodymyr Zelensky hinted at a potential new push in the counteroffensive last week, saying that Kyiv was "approaching a moment when relevant actions can gain pace because we are already going through some mine locations and we are demining these areas."

US Defense Secretary Lloyd Austin said Ukraine's western allies "can expect that [Ukraine] will continue to press."

Austin didn't comment on the status of the counteroffensive or specific battlefield details but said that Ukraine has been "preserving manpower and equipment," even as their forces deliberately work their way through minefields and other obstacles.

"Ukraine is well prepared and well trained to be successful, and as you heard me say last week, they fought hard, they've been working their way to get through the minefields and other obstacles, but they still have a lot of combat power," Austin said at a press conference in Papua New Guinea alongside the country's prime minister.

The New York Times was first to report on the commitment of additional Ukrainian forces to the counteroffensive.

'Careful offensive actions'

A step-up in the offensive would be welcome news for Ukraine and its allies after weeks of limited progress on the ground. The fighting has proved tougher than some anticipated despite Western nations donating billions of dollars' worth of aid to bolster Kyiv's military might and putting hundreds of soldiers through training.

Ukraine had hoped to expel a significant amount of Russian forces from Ukrainian soil and turn the tide of the war, but progress has so far been measured in meters instead of kilometers.

Some of the most strategic sections of the front line are guarded by multiple layers of defenses including minefields, trenches and anti-tank barriers, making it very difficult for the Ukrainians to break through. At the same time, Ukraine lacks air superiority, which means its troops are under frequent attacks from Russian warplanes as they try to push forward.

Ukraine's Defense Minister Oleksii Reznikov told CNN earlier this week that the offensive was going according to plan, but acknowledged that it was behind schedule. If Ukrainian forces can successfully puncture Moscow's sizeable defensive lines along Ukraine Sea of Azov coastline that links Crimea to Donbas, Reznikov said it would be "a good result" for Kyiv.

"We have to do it thinking about the lives of our soldiers instead of Russians. They're using the soldiers as cannon fodder," he told CNN in an interview.

Speaking in St. Petersburg on Thursday, Putin claimed that Ukraine was suffering "heavy losses."

"The enemy has very heavy losses of personnel -- over 200 people. Unfortunately, we did not do without losses, but the difference is colossal, at times -- more than ten times less than our enemy," he said.

Zelensky said last week that one reason for the slow progress of the counteroffensive was its later-than-planned start.

"We did have plans to start it in spring, but we didn't because frankly, we did not have enough munitions and armaments and not enough properly trained brigades, I mean, properly trained in these weapons," Zelensky told the Aspen Security Forum last week.

He said that the delayed start "provided Russia with the time to mine all our land and build several lines of defense."

"We didn't want to lose our people, our personnel and our servicemen, we didn't want to lose equipment and because of that, they were quite careful about the offensive actions," he added.

Kyiv says its Western backers are not pressuring the country for quick results. The United States recently supplied Ukrainian forces with cluster munitions and is now reportedly considering sending Army Tactical Missile Systems, guided missiles with a range of up to 300 kilometers (about 186 miles).

France and the United Kingdom have either delivered or pledged to supply Ukraine with long-range missiles.

At the same time, Ukraine has been investing heavily in drones development, signing up more than 40 Ukrainian companies to develop drones for deployment in the conflict. Ukrainian Prime Minister Denys Shmyhal said the production of drones has increased tenfold in Ukraine over the past year.

However, some Ukrainian military officials have recently noted that Russia's advantage in electronic warfare has blunted the effectiveness of drone operations.

Ukraine meanwhile has been struggling to repel a wave of Russian strikes against the southern city of Odesa, its air defenses unable to cope with the types of missiles that Moscow has used to pummel the region in recent days.