Ukrainian troops piled pressure on retreating Russian forces on Tuesday, pressing a counteroffensive that has produced major gains and a stunning blow to Moscow's military prestige.

It was not yet clear if the Ukrainian blitz in the northeast after months of little discernible movement could signal a turning point in the nearly seven-month war. But the country's officials were buoyant, releasing footage showing their forces burning Russian flags and inspecting abandoned charred tanks. In one video, border guards tore down a poster that read, "We are one people with Russia."

Momentum has switched back and forth before, and Ukraine's American allies, for one, were careful not to declare a premature victory since Russian President Vladimir Putin still has troops and resources to tap.

Still, the Kremlin struggled to respond to the defeat, its largest since its forces abandoned a botched attempt to capture Kyiv early in the war.

Late Monday, President Volodymyr Zelenskyy said his troops have retaken more than 6,000 square kilometers (2,300 square miles) – an area more than twice the size of Luxembourg – in a matter of weeks.

"Since the beginning of September and up to today, our fighters have liberated more than 6,000 square km of the territory of Ukraine in the south and in the east," Zelenskyy said in his nightly video. "The advances of our forces continue."

In the latest claim, Ukraine’s border guard services said the army took control of Vovchansk – a town just 3 kilometers (2 miles) from Russia seized on the first day of the war.

Meanwhile, Ukraine said Tuesday it aimed to liberate all of its territory after driving back Russian forces in the northeast of the country in a rapid offensive, but called on the West to speed up deliveries of weapons systems to back the advance.

Since Moscow abandoned its main bastion in northeastern Ukraine on Saturday, marking its worst defeat since the early days of the war, Ukrainian troops have recaptured dozens of towns in a stunning shift in battleground momentum.

While every individual claim of military success could not be verified, Russia acknowledged that it has withdrawn troops from areas in the northeastern region of Kharkiv in recent days.

In one indication of the blow sustained by Moscow, British intelligence said that one premier force, the 1st Guards Tank Army, had been "severely degraded" during the invasion and that "Russia’s conventional force designed to counter NATO is severely weakened. It will likely take years for Russia to rebuild this capability."

The retreat didn't stop Russia from pounding Ukrainian positions, however. Early Tuesday, it shelled the city of Lozova in the Kharkiv region, killing three people and injuring nine, said regional governor Oleh Syniehubov.

And Ukrainian officials said Russia kept up shelling around Europe’s largest nuclear facility, where fighting has raised fears of a nuclear disaster. The Nikopol area, which is across the Dnieper River from the Zaporizhzhia nuclear power plant, was shelled six times during the night but no injuries were immediately reported, said regional governor Valentyn Reznichenko.

Reports of chaos abounded as Russian troops pulled out – as well as claims that they were surrendering en masse. Ukraine officials have said they have captured so many soldiers that they are struggling to house them. The claim could not be immediately verified.

Ukrainian Deputy Minister of Defense Hanna Maliar said Kyiv is trying to persuade even more Russian soldiers to give up, launching shells filled with flyers ahead of their advance.

"Russians use you as cannon fodder. Your life doesn’t mean anything for them. You don’t need this war. Surrender to Armed Forces of Ukraine," the flyers read.

Strikes have also continued unabated on the city of Kharkiv, Ukraine's second-largest and one that has been hammered by artillery for months.

Zelenskyy specifically criticized Russia for targeting energy infrastructure in its attacks over the past days. "Hundreds and thousands of Ukrainians found themselves in the dark - without electricity. Houses, hospitals, schools, communal infrastructure ... sites that have absolutely nothing to do with the infrastructure of the armed forces of our country."

He said it could only point to one thing. "This is a sign of the desperation of those who contrived this war. This is how they react to the defeat of Russian forces in the Kharkiv region. They can’t do anything to our heroes on the battlefield."

The counteroffensive has provoked rare public criticism of Putin’s war. Meanwhile, some of its defenders in Russia played down the idea that the success belonged to Ukraine, blaming instead Western weapons and fighters for the losses.

"It’s not Ukraine that attacked Izium, but NATO," read a headline in the state-supported Komsomolskaya Pravda newspaper, referring to one of the areas where Russia said it has withdrawn troops.

Elsewhere, residents of a Russian village just across the border from Ukraine were evacuated after shelling by Ukrainian troops killed one person, according to Russia’s Tass news agency.

The report cited the head of the local administration in Logachevka, who said Ukrainian troops opened fire at a border checkpoint.

'Hanging by a thread'

Kremlin spokesperson Dmitry Peskov said there was no discussion of a nationwide mobilization to bolster the operation in Ukraine.

Criticism of Russia's leadership from online nationalist commentators who have demanded mobilization was an example of "pluralism," Peskov told reporters, adding that Russians as a whole continue to support President Vladimir Putin.

The Kremlin on Monday said the military operation would continue until acheiving its goals but sidestepped a question about whether Putin still had confidence in his military leadership.

Ukrainian officials say Russia has responded to Kyiv's battlefield successes by shelling power stations and other key infrastructure, causing blackouts in Kharkiv and elsewhere. Russia has blamed Ukraine for the blackouts.

Shelling around the Russian-held Zaporizhzhia nuclear power plant has sparked grave concerns about the risk of radioactive catastrophe. The U.N. atomic watchdog has proposed the creation of a protection zone around the nuclear plant, Europe's largest, and both sides are interested, IAEA chief said.

"We are playing with fire," Rafael Grossi told reporters. "We can not continue in a situation where we are one step away from a nuclear accident. The safety of the Zaporizhzhia power plant is hanging by a thread."