KYIV-Ukraine said Sunday its forces had begun moving into the key eastern town of Lyman, located in one of the four Ukrainian regions that Russia annexed, with President Volodymyr Zelensky pledging more areas would follow within the week.

The latest development -- a feature of Kyiv’s weeks-long counter-offensive against Moscow’s invasion -- comes as Germany’s defence minister made a surprise visit to Ukraine, but also amid accusations Russia killed 24 civilians in the eastern Kharkiv region.

The recapture of Lyman -- which Moscow’s forces pummelled for weeks to control this spring -- would mark the first Ukrainian military victory in territory that the Kremlin has claimed as its own and has vowed to defend by all possible means. Ukraine’s defence ministry announced its forces were “entering” Lyman in the eastern Donetsk region after the army said it had “encircled” several thousand Russian troops near the town.

The ministry posted a video of soldiers holding up a yellow and blue Ukrainian flag near a sign with the town’s name.

“Throughout this week, more Ukrainian flags have been raised in the Donbas,” Zelensky said in his evening address. “There will be even more in a week.” Shortly after Ukraine’s announcement on Lyman, Russia’s defence ministry said it had “withdrawn” troops from the town “to more favourable lines”.

With Russian losses mounting, experts have warned that President Vladimir Putin could turn to nuclear weapons to defend territory -- an option floated by a Putin ally. Chechen leader Ramzan Kadyrov said Russia should consider using low-yield nuclear weapons after Moscow’s troops were forced out of a Lyman. “In my personal opinion, more drastic measures should be taken, up to the declaration of martial law in the border areas and use of low-yield nuclear weapons,” Kadyrov said on his Telegram channel. Kadyrov governs Russia’s Muslim-majority Chechnya Republic with an iron fist. The developments came a day after Putin staged a grand Kremlin ceremony celebrating the annexation of the four Ukrainian territories. In a call with Ukrainian Defense Minister Oleksii Reznikov, Pentagon chief Lloyd Austin condemned what he called the “sham referenda”, according to a readout from his spokesman Saturday, and reiterated “the US will never recognize these illegal and illegitimate attempts at annexation.”

The four territories create a crucial land corridor between Russia and the Crimean Peninsula, also annexed by Moscow, in 2014. Together the five regions make up around 20 percent of Ukraine, where Kyiv in recent weeks has been clawing back territory.