KYIV, Sept 9 (Reuters) - Ukrainian forces were seizing an expanding area of previously Russian-held territory in the east in a "very sharp and rapid" advance, a Russian-installed regional official said on Friday, in a breakthrough that may mark a turning point in the war.
After keeping silent for a day, Russia effectively acknowledged that a section of its frontline had crumbled southeast of Ukraine's second-largest city Kharkiv.
"The enemy is being delayed as much as possible, but several settlements have already come under the control of Ukrainian armed formations," Vitaly Ganchev, head of the Russian-backed administration in the Kharkiv region, said on state television host Vladimir Solovyov's daily livestream.
Ganchev earlier said his administration was trying to evacuate civilians from cities including Izium, Russia's main stronghold and logistics base in the province near the front in eastern Ukraine.
Russia has taken control of around a fifth of Ukraine since starting its invasion in late February.
The Russian defence ministry released video of military vehicles speeding along a highway, saying they showed reinforcements rushing to defend the area. The Kremlin has declined to comment on the Ukrainian advance.
Ukrainian officials released a parade of videos showing soldiers raising flags and posing in front of street signs in villages and towns across part of previously Russian-held territory.
One viral image showed troops holding up a Ukrainian flag at a highway welcome sign for Kupiansk, previously more than 50 km inside Russia's front line. The city is an important target as the junction of several of the main railway lines supplying Russian troops at the front.
Ukraine kept independent journalists out of the area and Reuters could not confirm the images, but President Volodymyr Zelenskiy said troops had "liberated dozens of settlements" and reclaimed more than 1,000 square km (385 square miles) in the eastern Kharkiv and southern Kherson regions.
Western military analysts said the advance could shut the supply lines Moscow has relied on to sustain its force in eastern Ukraine, and potentially leave thousands of Russian troops encircled.
SUCCESS
Such rapid advances have largely been unheard of since Russia abandoned its assault on the capital Kyiv in March, shifting the war mainly into a relentless grind along entrenched front lines.
"We see success in Kherson now, we see some success in Kharkiv and so that is very, very encouraging," U.S. Defense Secretary Lloyd Austin told a news conference with his Czech counterpart in Prague.
The Ukrainian general staff said early on Friday that retreating Russian forces were trying to evacuate wounded personnel and damaged military equipment near Kharkiv.
"Thanks to skilful and coordinated actions, the Armed Forces of Ukraine, with the support of the local population, advanced almost 50 km in three days."
Tens of thousands of people have been killed, millions have been driven from their homes and Russian forces have destroyed entire cities since Moscow began what it calls a "special military operation" to "disarm" Ukraine. The Kyiv government and its Western allies accuse Russia of an imperial-style war of aggression.
In the latest reported strike on civilians, Ukrainian officials said Russia had fired across the border, hitting a hospital in the northeastern Sumy region on Friday morning, destroying the building and wounding people. Reuters could not independently confirm the report.
Russia denies intentionally targeting civilians.
The centre of Kharkiv, which has been regularly bombarded by Russia, was hit by Russian rocket fire, wounding 10 people, including three children, Governor Oleh Synehubov said. Rockets hit a children's arts centre and a school, as well as private homes, mayor Ihor Terekhov wrote on Telegram.
BREAKTHROUGH
The Ukrainians broke through in the east a week after Kyiv announced the start of a long-awaited counter-offensive hundreds of kilometres away at the other end of the front line, in the southern province of Kherson.
Ukrainian officials said Russia moved thousands of troops south to respond to the Kherson advance, leaving other parts of the front line exposed.
"We found a weak spot where the enemy wasn't ready," presidential adviser Oleksiy Arestovych said in a video posted on YouTube.
Russia's state news agency RIA quoted Russian-appointed Kherson authorities as saying some Ukrainian troops were captured during the counterattack and some Polish tanks they were using were destroyed. Reuters could not verify those reports.
Less information so far has emerged about the campaign in the south, with Ukraine keeping journalists away and releasing few details.
Ukraine has been using new Western-supplied artillery and rockets to hit Russian rear positions there, with the aim of trapping thousands of Russian troops on the west bank of the wide Dnipro River.
Arestovych acknowledged progress in the south had not yet been as swift as in the east.
Separately, the U.N. nuclear watchdog said on Friday that shelling has destroyed power infrastructure in the southern Ukrainian city of Enerhodar where staff operating the Russian-held Zaporizhzhia nuclear power station live, posing a growing threat to the plant.
The plant's offsite power lines, vital lines of defence against potential nuclear meltdown, have already been cut and the shelling at Enerhodar has caused a lasting blackout there.
"This is an unsustainable situation and is becoming increasingly precarious. Enerhodar has gone dark. The power plant has no offsite power. And we have seen that once infrastructure is repaired, it is damaged once again," International Atomic Energy Agency chief Rafael Grossi said in a statement.
