The Russian Defense Ministry has acknowledged its forces retreated from positions north of the city of Bakhmut in Ukraine's eastern Donetsk region that for months has been the scene of heavy fighting.
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Defense Ministry spokesman Igor Konashenkov said at a news briefing on May 12 that Russian forces had "occupied a new frontier" at the Berkhivske reservoir.
Konashenkov said Ukraine had launched an assault with more than 1,000 troops and up to 40 tanks after advancing the day before in the Soledar direction "along the entire line of contact" with a length of more than 95 kilometers.
The retreat allows Ukrainian troops to control the road to the town of Chasiv Yar, the only highway currently supplying Ukrainian forces in Bakhmut.
The gains, if confirmed, would be the biggest for Ukraine in six months. It is also unclear if the developments were part of a long-planned counteroffensive, which the head of Russia's Wagner mercenary group, Yevgeny Prigozhin, said on May 11 had already begun.
Prigozhin, Russian military bloggers, and the Ukrainian Army had previously reported about the retreat. Prigozhin acknowledged it in an audio message, saying that what Konashenkov described "unfortunately is called a rout and not a regrouping."
In a separate video message, he said the Ukrainians had seized high ground overlooking Bakhmut and opened the main highway leading into the city from the west. This could lead to the encirclement of his mercenaries inside the city, Prigozhin said.
There also were reports of Ukrainian advances to the south, suggesting a coordinated push by Kyiv to encircle Russian forces in Bakhmut.
Russian-installed officials in Luhansk on May 12 reported two powerful explosions in the city, which is far from Ukrainian positions. Photos published on Telegram showed a column of black smoke rising over the city.
Officials said on Telegram that two Ukrainian-made Grom tactical missiles damaged or destroyed buildings at two disused factories and blew out the windows of five houses.
The Moscow-backed administrator of the Luhansk region, Leonid Pasechnik, said six children had been injured. Viktor Volodatsky, a member of the State Duma, the lower house of Russia's parliament, was also injured, according to other officials.
There was no comment from the Ukrainian side, and the claims could not be independently verified.
Earlier on May 12, Kyiv claimed modest advances in the Bakhmut area. Deputy Defense Minister Hanna Malyar said on Telegram that Ukrainian forces had advanced 2 kilometers without ceding any ground elsewhere to the Russians.
In turn, Russia's Defense Ministry said its troops had fought off several Ukrainian attempts to overrun their positions while pounding the Bakhmut-Avdiyivka-Maryinka front line that has seen the fiercest fighting over the past several months.
Neither side's claims could be independently verified.
Ukrainian President Volodymyr Zelenskiy vowed again to defeat the Russians and thanked everyone involved in driving them out of Ukrainian territory.
"The occupiers of Ukraine are already internally ready for defeat. They have already lost this war in their minds. We must put pressure on them daily so that their sense of defeat turns into their escape, their mistakes, their losses," he said on May 12 on Twitter.
"We are preparing to add even more weapons for you, warriors, to have even more opportunities to defeat the Russian aggressor and restore peace. I'm thankful to everyone who helps Ukraine," he added.
Ukraine's preparations for a counteroffensive received an important boost on May 11 when Britain announced it was supplying long-range cruise missiles to Ukraine, giving Kyiv the capability to hit Russian troops well behind front lines. Storm Shadow missiles have a range of more than 250 kilometers.