Ukrainian artillery has struck a headquarters of Russia's shadowy Wagner paramilitary group of mercenaries in eastern Ukraine, reports say.
The extent of damage to the military base of the group - which has been linked to war crimes - is not clear.
Luhansk's governor claims its secret location was revealed after a Russian journalist shared its address.
Last week, pro-Kremlin correspondent Sergei Sreda posted a photo on Telegram of the base with its apparent address.
The image, shared on the social media site but since deleted, shows five people in military uniform with a street sign in Popasna, Luhansk visible in the top left corner.
While the BBC has not been able to confirm the connection between the photo and the strike, Governor Serhiy Hayday wrote on Telegram that Ukrainian forces "hit an enemy HQ whose whereabouts were established thanks to a Russian journalist".
"This time, the successful strike destroyed the Wagner PMC HQ in Popasna yesterday," he said. He added that "the number of dead is being clarified".
The Wagner group was deployed to Crimea and Ukraine's Donbas region in 2014, when Kremlin-backed forces ousted Ukrainian troops from areas they later declared to be part of Russia.
Wagner units have also been sent to Syria, Libya, Mali and the Central African Republic.
The Kremlin does not acknowledge Wagner's existence, but Western intelligence links the group to Yevgeny Prigozhin - nicknamed "Putin's chef" because his catering business has long helped President Vladimir Putin and the armed forces.
Mr Prigozhin, like many other Russian officials, is subject to Western sanctions.
Wagner is known as a PMC - private military company. But they are state-sponsored mercenaries who act in the Kremlin's interests, Western experts say. They have been accused of repeated war crimes and human rights abuses.
The Ukrainian daily Ukrainska Pravda reports that a pro-Kremlin Russian journalist, Sergei Sreda, revealed the Wagner HQ in a Telegram post on 8 August. He posted photos of his visit to the HQ and a sign in one of them identified its address as Mironovskaya 12, Popasna.
The post was later removed, but copies are circulating on social media.
Popasna lies just south of Severodonetsk in Luhansk region, which Russian forces now entirely control, following months of bitter fighting and huge destruction.
Another pro-Kremlin war reporter, called Kotenok, wrote on Telegram: "A strike was carried out on one of the Wagner PMC locations in Popasna. Sources in Donbas confirm that. Probably 'Himars'. Ukrainian sources report the death of Prigozhin - we don't confirm that."
Ukrainian forces now have several of the US Himars (High Mobility Artillery Rocket System), which fire rockets capable of hitting Russian targets far behind the front lines.
Ukrainian MP Oleksiy Honcharenko wrote on Facebook: "There is no more Wagner HQ in Popasna. Thank you, Himars and the Armed Forces of Ukraine!"
Some Ukrainian reports implicate Wagner in the Olenivka prison fire on 29 July which killed dozens of Ukrainian prisoners-of-war. Russia blamed Ukrainian forces, saying they had shelled the prison, about 20km (12.5 miles) from Donetsk. The circumstances of the inferno remain unclear.