Ukraine have claimed a major breakthrough by breaching the first and strongest of several Russian defense lines in the southern Zaporizhzhia region.

Gen. Oleksandr Tarnavskiy, leading the southern counteroffensive, spoke several days after Kyiv declared a strategic victory by recapturing the southern village of Robotyne.

The interview came as President Volodymyr Zelenskyy has increasingly vocally dismissed criticism of Kyiv's counteroffensive, that has made slower gains than expected.

"We are now between the first and second defensive lines," Tarnavskiy said.

"In the center of the offensive, we are now completing the destruction of enemy units that provide cover for the retreat of Russian troops behind their second defensive line."

Heavily mined territory had slowed Ukrainian troops, saying that sappers had cleaned a route by foot and at night.

Tarnavskiy told The Guardian that Moscow's forces "just stood and waited for the Ukrainian army."

The paper quoted him as saying that Kyiv's forces are now back on vehicles and that Russia has redeployed troops to the area.

"The enemy is pulling up reserves, not only from Ukraine but also from Russia. But sooner or later, the Russians will run out of all the best soldiers. This will give us an impetus to attack more and faster," Tarnavskiy said. "Everything is ahead of us."

Under Tarnavskiy's command, Ukrainian troops liberated the southern city of Kherson last year.

He said that when Kyiv launched its counteroffensive in June, it "spent more time than we expected on demining territories."

"Unfortunately, the evacuation of the wounded was difficult for us. And this also complicated our advance," he said.

Tarnvaskiy admitted difficult losses for Kyiv.

"The closer to victory, the harder it is. Why? Because, unfortunately, we are losing the strongest and best," he said.

"So now we have to concentrate on certain areas and finish the job. No matter how hard it is for all of us."

On Saturday, Zelensky said shrugged off criticism of the counteroffensive.

"Despite everything and no matter what anyone says, we are advancing, and that is the most important thing," he said on Telegram.

"We are on the move."