NATO Secretary-General Jens Stoltenberg said Thursday that Ukraine is making progress with a counteroffensive started in June to reclaim territory seized by Russia.

He pointed out, however, that the process was slow due to Russian fortifications and minefields.

"The Ukrainians are gradually gaining ground ... They have been able to breach the defensive lines of the Russian forces, and they are moving forward," Stoltenberg told lawmakers in remarks at the European Parliament.

Since launching its offensive, Kyiv has struggled to break through entrenched Russian lines and has faced growing criticism in Western media of concentrating forces in the wrong places.

With Moscow's stretched military resources and dissent in the ranks, however, both sides have measured recent successes by taking control of tiny villages or small pockets of land.

Stoltenberg said it had to be expected that the offensive would be advancing only slowly.

"No one ever said that this was going to be easy," he noted. "Hardly any time in history we have seen more mines on the battlefield than we are seeing in Ukraine today. So it was obvious that this was going to be extremely difficult."

Ukrainian officials have said in the past week that their forces have managed to push past Russia's first line of defenses but now confronted further lines in areas where Moscow has had time to build up fortifications and minefields.

"They are making progress. Not perhaps as much as we hoped for but they are gaining ground gradually," said the NATO chief. "Some hundred meters per day, meaning that when the Ukrainians are gaining ground, the Russians are losing ground."

Praising the Ukrainian forces for their achievements on the battlefield, he added: "The starting point is that the Russian army used to be the second strongest in the world. And now the Russian army is the second strongest in Ukraine. That's quite impressive by Ukrainians."

Blinken hails resilience

Meanwhile, U.S. Secretary of State Antony Blinken praised Thursday Ukraine's strength in the face of the Russian invasion during a visit to the Chernigiv region, which was occupied by Moscow at the beginning of the war.

Blinken visited a school's basement in Yagidne, where Russian troops kept dozens of villagers including elderly people and children captive.

"This is just one building ... (but) this is a story we've seen again and again," Blinken said.

"But we are also seeing something else that's incredibly powerful ... the extraordinary resilience of the Ukrainian people."

Russian forces had seized parts of the Chernigiv region, including Yagidne, soon after the beginning of the war.

They withdrew after about a month, and Yagidne was recaptured by Ukrainian forces on March 30, 2022.

But the Russian army left towns and land destroyed and heavily mined.

Blinken, who arrived in Kyiv on an unannounced visit Wednesday, said up to a third of Ukraine's territory was now dealing with mines or unexploded ordnance.

"But Ukrainians are coming together to get rid of the ordnance, to get rid of the mines, and to literally recover the land," Blinken said.

The top U.S. official said Washington was "proud" to support Ukraine's efforts to "take on the aggression as they recover as they rebuild."