Ukraine said its troops have marched farther east into territory recently abandoned by Russia, paving the way for a potential assault on Moscow's occupation forces in the Donbas region as Kyiv seeks more Western arms.
In a sign of nervousness from a Moscow-backed administration in Donbas about the success of Ukraine's recent offensive, its leader called for urgent referendums on the region becoming part of Russia.
"The occupiers are clearly in a panic," Ukraine's President Volodymyr Zelensky said in a televised address late on Monday, adding that he was now focused on "speed" in liberated areas.
"The speed at which our troops are moving. The speed in restoring normal life," Mr Zelensky said.
The Ukrainian leader also hinted he would use a video address to the United Nations General Assembly on Wednesday to call on countries to accelerate weapons and aid deliveries.
"We are doing everything to ensure Ukraine's needs are met at all levels - defence, financial, economic, diplomatic," Mr Zelensky said.
Serhiy Gaidai, Ukrainian governor of Luhansk, a province in the Donbas now under control of Russian troops, said Ukraine's armed forces had regained complete control of the Luhansk village of Bilohorivka and were preparing to fight to retake the entire province.
"There will be fighting for every centimetre," Mr Gaidai wrote on Telegram. "The enemy is preparing their defence. So we will not simply march in."
In another important milestone for the counter-offensive in the northeastern Kharkiv region, Ukraine's armed forces said troops had crossed the Oskil River over the weekend.
The river flows south into the Siversky Donets, which snakes through the Donbas, the main focus of Russia's invasion.
Further beyond lies Luhansk, a base for Russia's separatist proxies since 2014 and fully in Russian hands since July after some of the war's bloodiest battles.
A Russia-backed separatist official in Donetsk, the other province in Donbas, said 13 people were killed in artillery shelling on Monday in the city of Donetsk.
Reuters could not independently verify either side's battlefield reports.
Ukraine is still assessing what took place in areas that were under Russian control for months before a rout of Russian troops dramatically changed the dynamic of the war earlier this month.
At a vast makeshift cemetery in woods near the recaptured town of Izium, Ukrainian forensic experts have so far dug up 146 bodies buried without coffins, Kharkiv regional governor Oleh Synehubov said on Monday.
Some 450 graves have been found at the site, Mr Zelensky has said.
Fanning out in groups beneath the trees, workers used shovels to exhume the partially decomposed bodies, some of which locals said had lain in the town streets long after they died before being buried.
The government has not yet said how most of the people died, though officials say dozens were killed in the shelling of an apartment building, and there are signs others were killed by shrapnel.
According to preliminary examinations, four showed signs of torture, with their hands tied behind their backs, or in one case a rope tied round their neck, Serhiy Bolvinov, the head of investigative police in the Kharkiv region, told Reuters at the burial ground.
Mr Bolvinov said the great majority of the bodies appeared to be civilians. Locals have been identifying their dead by matching names to numbers on flimsy wooden crosses marking the graves.
"Soldiers had their hands tied, there were signs of tortureon civilians," Mr Bolvinov said. Ukraine says 17 soldiers were in amass grave at the site.
Reuters could not corroborate Ukraine's allegations of torture.
The Kremlin denied on Monday that Russia was to blame for atrocities that Ukraine says it has uncovered in the recaptured territory.
"It's a lie, and of course we will defend the truth in this story," Kremlin spokesman Dmitry Peskov said, comparing the allegations to incidents earlier in the war where Russia claimed without evidence that atrocities were staged by Ukrainians.
Alarm over nuclear plant
Ukraine accused Russian forces on Monday of shelling near the Pivdennoukrainsk nuclear power plant in the country's southern Mykolaiv region.
A blast occurred 300 metres away from the reactors and damaged power plant buildings shortly after midnight on Monday, Ukraine's atomic power operator Energoatom said in a statement.
The reactors were not damaged and no staff were hurt, it said, publishing photographs showing a huge crater it said was caused by the blast.
"Russia endangers the whole world. We have to stop it before it's too late," Mr Zelensky said in a social media post.
The strikes will add to global concern over the potential for an atomic disaster, already elevated by fighting around another Ukrainian nuclear power plant in the south, Zaporizhzhia, captured by Russian forces in March.
Moscow has ignored international calls to withdraw and demilitarise it.
In a new setback at Zaporizhzhia, the IAEA said a power line used to supply the plant was disconnected on Sunday, leaving it without backup power from the grid.