Ukrainian leader Volodymyr Zelenskiy has spoken of "good news" on the battlefields of eastern Ukraine, saying his army has retaken towns and villages from Russia in what analysts say looks like a deep thrust behind Russian lines.
In his daily late night address on Wednesday, President Zelenskiy said he had received news his forces had liberated a slew of settlements in the Kharkiv region in a counter offensive that some Western analysts suggested had seen Kyiv recapture around 400 square kilometres of territory.
"This week we have good news from Kharkiv Oblast. All of you have most likely seen reports about the recent activities of Ukrainian defenders. And I think every (Ukrainian) citizen feels proud of our warriors," said Zelenskiy.
Kharkiv region borders Russia and its main city, Kharkiv, has for months been struck by Russian missiles after Moscow failed to take it in the early stages of its February 24 invasion.
In a sign that the situation in the area was still highly fluid though, Zelenskiy said it was too early to name the recaptured towns and villages while thanking two airborne brigades and a mechanised brigade for what he called their bravery.
Such a thrust, if confirmed and the gains are held, would be a significant boost for Kyiv, which is keen to show its Western backers that it can change the facts on the ground by force and deserves continued financial and weapons support.
There is additional pressure on Kyiv to demonstrate that before winter sets in amid threats by Russian President Vladimir Putin to halt all energy shipments to Europe if Brussels goes ahead with a proposal to cap the price of Russian gas.
In a boost for Kyiv, US Defence Secretary Lloyd Austin said on Thursday that President Joe Biden had approved an additional $US675 million in weapons to Ukraine as he and other defence ministers met in Germany to discuss how to continue supporting Ukraine in the long-term.
The Institute for the Study of War (ISW), a Washington-based think tank, which follows the war day-by-day, said the Ukrainian military looked to have made significant progress on Wednesday.
"Ukrainian forces likely used tactical surprise to advance at least 20km into Russian-held territory in (the) eastern Kharkiv Oblast (region) on September 7, recapturing approximately 400 square kilometres of ground," the ISW said.
Russia has confirmed fighting in the area but has not confirmed any territorial losses, though unverified social media accounts run by Russian military experts have suggested Moscow did suffer setbacks and will need to urgently reinforce.
Heavy fighting was also reported on Thursday in areas near the Russian-held Zaporizhzhia nuclear power station in Ukraine after Kyiv warned it might have to shut down the plant to avoid disaster.
Putin said in a speech on Wednesday that Russia would not lose what he calls its "special military operation" in Ukraine, an exercise he has described as an attempt to preemptively protect his country's security against an expanding NATO.
Asked about the war's progress, Putin said: "We have not lost anything and will not lose anything."
He also threatened to halt all energy supplies to Europe if Brussels adopted a proposed price cap on Russian gas, the latest Western step to deprive the Kremlin of funds to finance the war.