KHARKIV, Ukraine (AP) — Ukrainian troops continued to pile unrelenting pressure on retreating Russian forces on Tuesday, seeking to hold on to their sudden momentum that has produced major territorial gains.

Fresh yellow-and-blue flags fluttered from the tallest buildings left in partly destroyed towns around Ukraine’s second city, Kharkiv, while Ukrainian soldiers inspected charred Russian tanks left along the way.

“From the beginning of September until today, our soldiers have already liberated more than 6,000 square kilometers of the territory of Ukraine — in the east and south. The movement of our troops continues,” said Ukrainian President Volodymyr Zelenskyy in his nightly address late Monday.

Many of the claims of military success could not be independently verified.

Rubbing salt into Russia’s wounds, British intelligence said that one of Moscow’s premier forces, 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.

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. Continued shelling has left Europe’s largest nuclear facility in a precarious position.

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.”

Ukrainian military intelligence said Russian troops were surrendering en masse. A Ukrainian presidential adviser said there were so many prisoners of war that the country was running out of space to accommodate them.

The counteroffensive left the Kremlin struggling for a response to its largest military defeat in Ukraine since Russian forces pulled back from areas near Kyiv after a botched attempt to capture the capital early in the invasion.

The Russian Defense Ministry acknowledged the setback in a map that showed its troops pressed back along a narrow patch of land on the border with Russia — a tacit admission of big Ukrainian gains.

It was not yet clear if the Ukrainian blitz could signal a turning point in the war. Momentum has switched back and forth before, but rarely with such a big and sudden swing.

Some in Russia blamed 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.