KYIV, Ukraine -- Ukraine said Saturday 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Shortly after Ukraine's announcement on Lyman, Russia's defence ministry said it had "withdrawn" troops from the town "to more favourable lines".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.
Elsewhere in the south Saturday, German Defence Minister Christine Lambrecht visited the port city of Odessa.
Kyiv has been urging her country to send battle tanks to aid in its counter-attack, but the German government has so far refused.
Civilians gunned down
Also on Saturday, Ukrainian officials accused Russia of killing 24 civilians, including 13 children, in an attack on a road convoy near a recently recaptured town in Kharkiv.
Ukrainian troops on Friday had shown AFP reporters a group of vehicles riddled with bullet holes and several corpses in civilian clothes, a short distance east of the recently recaptured town of Kupiansk.
A Ukrainian official said the death toll of a Russian attack on a separate civilian convoy near the city of Zaporizhzhia on Friday had risen to 30 civilians and one police officer killed.
Kyiv also called for the immediate release of the chief of the Moscow-held Zaporizhzhia nuclear power plant, condemning his "illegal detention" by the Russians.
Ihor Murashov was leaving the plant Friday when he was detained and "driven in an unknown direction" while blindfolded, Ukraine's nuclear agency Energoatom said.
Zaporizhzhia -- Europe's largest nuclear energy facility -- has been at the centre of tensions with Moscow and Kyiv accusing each other of strikes on and near the plant, raising fears of an atomic disaster.
'Illegitimate' annexation
Following Friday's annexation, Washington announced "severe" new sanctions against Russian officials and the defence industry, and said G7 allies support imposing "costs" on any nation backing annexation.
Zelensky urged the US-led military alliance NATO to grant his country fast-track membership.
He also vowed never to hold talks with Russia as long as Putin was in power.
NATO chief Jens Stoltenberg slammed the annexation as "illegal and illegitimate" but remained non-committal after Ukraine said it was applying to join the Western alliance.
Turkey said Saturday Russia's annexation was a "grave violation of the established principles of international law".
Despite Putin's warnings prior to the annexation that he could use nuclear weapons to defend the captured territories, Ukraine's Foreign Minister Dmytro Kuleba said Kyiv would "continue liberating our land and our people".
Kuleba also said Ukraine brought the annexations to the International Court of Justice and urged the Hague-based court to hear the case "as soon as possible".