Russia said on Tuesday it was carrying out "massive strikes" across the Ukrainian front line and accused Ukrainian soldiers of abusing civilians in territories recaptured in a dramatic counter-offensive.

Moscow's retaliation came after it was forced to pull back its troops from swathes of the northeast, particularly in the Kharkiv region, following Kyiv's lightning assault to wrest back terrain.

The territorial shifts marked one of Russia's biggest setbacks since its troops were repelled from Kyiv in the earliest days of the nearly seven-month war, yet Moscow signalled it was no closer to agreeing to a negotiated peace.

"Air, rocket and artillery forces are carrying out massive strikes on units of the Ukrainian armed forces in all operational directions," the Russian defence ministry said in its daily briefing on the conflict.

"High-precision" strikes have also been launched on Ukrainian positions around Sloviansk and Konstantinovka in the eastern Donetsk region, it added.

The Kremlin accused Kyiv's army of abusing civilians in territory it had recaptured.

President Vladimir Putin's spokesman said that in the Kharkiv region, reports were emerging of "outrageous" treatment of civilians.

"There are a lot of punitive measures... people are being tortured, people are being mistreated and so on," Dmitry Peskov told journalists.

Russia's allegations came after Ukrainian authorities claimed to have found four bodies of civilians with "signs of torture" in the recaptured eastern village of Zaliznychne.

Residents reported that Russian troops had killed villagers, the regional prosecutor's office said, announcing a war crime probe.

Ukrainian forces launched their counter-offensive in early September, seemingly catching Russia's military off guard.

Images posted by the Ukrainian military showed crates of munitions and military hardware scattered across territory abandoned by Russian forces.

Around the north-eastern town of Balakliya, AFP journalists saw evidence of fierce battles, with buildings destroyed or damaged and streets mostly deserted.

By Monday, President Volodymyr Zelensky said that Ukraine's forces have retaken a total of 6,000 square kilometres from Russian control.

In the northeast, dozens of areas including the cities of Izyum, Kupiansk and Balakliya, have been retaken, Ukraine said.

Ukraine has also claimed significant gains in the southern Kherson region, where the Ukrainian army also said it had recaptured 500 square kilometres.

US Secretary of State Antony Blinken assessed that the Ukrainians had made "significant progress", due to their resilience as well as US support.

"It's too early to tell exactly where this is going. The Russians maintain very significant forces in Ukraine as well as equipment and arms and munitions.

"They continue to use it indiscriminately against not just the Ukrainian armed forces but civilians and civilian infrastructure as we've seen," Blinken said on Monday. (AFP)