A Russian missile demolished an apartment block in a Ukrainian region that Moscow says it has annexed, killing at least seven people, as discontent mounted within Russia about the handling of the war by the top brass.

The missile attack on the city of Zaporizhzhia in the southern region of the same name left some people buried under rubble, the regional governor said, and was a reminder of Moscow's ability to strike targets even at a time when its forces have been pushed back in the south and east.

There was no immediate comment from Russia, whose invasion of Ukraine has begun to unravel after a Ukrainian counter offensive in which thousands of square miles of territory have been retaken since the start of September, including dozens of settlements in recent days.

Thousands of Russian troops have retreated after the frontline crumbled, first in the northeast, and, since the beginning of this week, also in the south.

Public criticism of Russia's top military officials, once taboo, is growing.

Today, a Russian-installed official in occupied Ukraine openly mused about the idea of Sergei Shoigu, the defence minister and an ally of Putin, shooting himself due to the shame of his military failures.

"Indeed, many say: if they were a defence minister who had allowed such a state of affairs, they could, as officers, have shot themselves," Kirill Stremousov, the deputy head of the Russian-installed administration in Ukraine's Kherson region, said in a video.

There was no immediate comment from the defence ministry.

Discontent has begun to bubble up among even loyalist state TV hosts.

"Please explain to me what the general staff's genius idea is now?" Vladimir Solovyov, one of the most prominent Russian talk show hosts, said on his livestream channel.

"Do you think time is on our side? They (the Ukrainians) have hugely increased their amount of weapons... But what have you done in that time?"

Rubble, smoke and dust

Images of the aftermath of Thursday's missile strike showed a gaping, rubble-strewn hole where a five-storey apartment block used to stand next to a wine shop.

Reuters reporters saw firefighters bringing a father and son down a ladder and talking to an older man still trapped under rubble.

Eduard, a 49-year-old man who survived the attack, said he was woken at around five in the morning by a strong explosion.

"The room filled with smoke and dust. I jumped up to go see what had happened," he said.

Twelve people were wounded, including a three-year-old child, and five were still under the rubble, governor Oleksandr Starukh said.

In an online address to new security and energy co-operation forum the European Political Community, Ukraine's President Volodymyr Zelensky accused Russia of deliberately targeting the same spot twice in succession.

"In Zaporizhzhia, after the first rocket strike today, when people came to pick apart the rubble, Russia conducted a second rocket strike. Absolute vileness, absolute evil."

Moscow says it does not deliberately target civilians.

Russian President Vladimir Putin at the Novo-Ogaryovo state residence outside Moscow today

In remarks to Australia's Lowy Institute, Zelensky said NATO should launch preventive strikes on Russia to preclude its use of nuclear weapons.

Kremlin spokesman Dmitry Peskov denounced the comments as "an appeal to start yet another world war with unpredictable, monstrous consequences," according to RIA news agency.

The Zaporizhzhia missile attack came a day after Putin signed a law to incorporate four partially occupied Ukrainian regions into Russia, including Zaporizhzhia, in Europe's biggest attempted annexation since World War Two.

Kyiv called the new law the act of a "collective madhouse".

Russia moved to annex the regions after holding what it called referendums – votes denounced by Kyiv and Western governments as illegal and coercive.

A destroyed school recaptured from Russian forces in the village of Verbivka, Kharkiv Oblast, Ukraine

'Scorched earth'

In the Kharkiv region in the northeast, a Ukrainian general said that Kyiv's forces had advanced up to about 55km over the last two weeks.

In the eastern Donetsk region, the Ukrainian General Staff said Russian troops had blown up a dam near the city of Sloviansk as they withdrew, inundating the nearby town of Raihorodok.

Reuters could not confirm the report independently.

Russian forces remained dug in around the nearby battered city of Bakhmut.

Ukrainian multiple rocket launchers and artillery exchanged near constant fire today with Russian forces who have pummelled the area for months, prompting the flight of much of the pre-war population of 70,000.

Ukraine also said it downed five drones, three of them made in Iran, without saying where the strikes took place.

Ukraine has reported a spate of attacks with Iranian-made Shahed-136 drones in recent weeks.

Iran denies supplying the drones, while the Kremlin has not commented.

Reuters could not immediately verify the battlefield reports.