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The Kremlin is also said to be turning to retired personnel as Russia’s losses mount after more than six weeks of war - with Ukraine claiming on Monday that 19,500 invading troops had been killed and 725 tanks destroyed.
After failing to capture Kyiv, Russia has repositioned forces to the south and east of Ukraine in what the government in Kyiv and its allies fear is a prelude to a new onslaught on the Donbas region.
"We have to assume that the brutality of the war in Ukraine will increase," said Austria's Foreign Minister Alexander Schallenberg, who said Russian President Vladimir Putin appeared to be "throwing his entire military might into the east".
To the west, French police officers arrived in Ukraine on Monday to investigate the alleged atrocities uncovered when Russia abandoned towns it had occupied near Kyiv.
The European Union's 27 foreign ministers meanwhile held talks with the top prosecutor of the International Criminal Court as they prepare possible war crimes charges against the Russian leadership.
In the village of Buzova outside Kyiv, local officials said bodies showing “evidence of execution” had been discovered following the Russian withdrawal.
Buzova is near Bucha, another town where atrocities were discovered and where satellite images undermined Russia's claims that the massacre was staged. Ukraine's prosecutor general Iryna Venediktova says 1,222 bodies have been discovered in areas from which the Russian army retreated.
Britain's defence ministry said in a regular intelligence update that Russia's reliance on unguided bombs increased the risk to civilians and that Moscow's forces could use phosphorous munitions in their battle for the port city of Mariupol.
White phosphorus is used for illumination at night or to create a smokescreen, but when it is deployed as a weapon it causes horrific burns.
The ministry separately said that Russia was trying to recruit from the unrecognised Transnistria region of Moldova, which borders of Ukraine, and that personnel discharged from military service since 2012 were also being used to bolster troop numbers.
It said Ukraine has beaten back several assaults by Kremlin forces in the Donetsk and Luhansk regions, resulting in the destruction of Russian tanks, vehicles and artillery.
Western powers also believe Russia has turned to the Wagner mercenary group, a private army widely believed to be close to the Kremlin, to supplement its forces in eastern Ukraine.
Russian troops have long been stationed in Transnistria, whose status has been unresolved since a war in the early 1990s that followed the collapse of the Soviet Union.
The majority Russian-speaking area is outside the control of the Moldovan authorities and Ukrainian President Volodymyr Zelenskyy has compared the Kremlin’s intervention there with its support for separatists in eastern Ukraine.
Oleksandr Shputun, a spokesman for Ukraine’s military, expressed concern on Monday that Russian troops could stage “provocative actions” in Transnistria.
A false-flag operation such as this could be used “to accuse Ukraine of aggression against a neighbouring state”, he said.
Britain also blamed Russia for encouraging unrest in Bosnia and Herzegovina as it announced sanctions against two politicians there for advocating the break-up of the country, the first such measures affecting the Balkan country.
The UK's Foreign Office said Milorad Dodik and Zeljka Cvijanovic had been "emboldened by Russia’s undermining of the international rules-based system" to push for the secession of the Russia-friendly Republika Srpska.
"Encouraged by Putin, their reckless behaviour threatens stability and security across the Western Balkans," said Foreign Secretary Liz Truss.