* Ukraine says its forces advance to eastern bank of Oskil
* Governor says forces enter towns near Lysychansk
* Separatist leader calls for urgent referendum to join Russia
* Forensic workers exhume 146 bodies in Izium
* Ukraine says Russia strikes Pivdennoukrainsk nuclear plant
By Tom Balmforth
IZIUM, Sept 19 (Reuters) - Ukraine extended its hold on recently recaptured territory on Monday as troops marched farther east into areas abandoned by Russia, paving the way for a potential assault on occupation forces in the Donbas region.
In a sign of nervousness from a Moscow-backed administration in Donbas about the success of Ukraine's 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 Zelenskiy said in a regular televised address, 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," said Zelenskiy, who also hinted he would use a video address to the U.N. General Assembly this week to call on the international community to get weapons and aid to Ukraine more quickly.
Serhiy Gaidai, Ukrainian governor of Luhansk, a province in the Donbas now under control of Russian troops, said Ukrainian forces had regained control of the town of Kreminna and the village of Bilohorivka close to the city of Lysychansk, which fell after weeks of grinding battles in July.
"Luhansk region is right next door. De-occupation is not far away," he wrote on Telegram.
Ukrainian troops also crossed the Oskil River at the weekend, the Ukrainian Armed Forces wrote on Telegram late on Sunday, in another important milestone for the counter-offensive in the northeastern Kharkiv region. 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, Zelenskiy 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.
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 torture on civilians," Bolvinov said. Ukraine says 17 soldiers were in a mass 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 (yards) away from the reactors and damaged power plant buildings shortly after midnight, 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," Zelenskiy 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.
Russia's rapid losses over the past few weeks have shaken a Kremlin public relations campaign that has never veered from the line that the "special military operation" is "going to plan".
Alla Pugacheva, 73, Russia's most celebrated pop diva since the Soviet era, became by far the biggest mainstream cultural figure to oppose the war, with a post on Instagram denouncing "the death of our guys for illusory goals that are turning our country into a pariah and worsening the lives of our citizens".
In London, Ukrainian Prime Minister Denys Shmyhal and First Lady Olena Zelenska attended the funeral of Britain's Queen Elizabeth II. Russia was banned from the ceremony.
(Reporting by Reuters bureaus; Writing by David Brunnstrom, Stephen Coates and Peter Graff; Editing by Shri Navaratnam, Lincoln Feast, Mark Heinrich and Cynthia Osterman)