The UK's Ministry of Defence (MOD) fears that Vladimir Putin's forces could use white phosphorus munitions in the bombardment of the besieged Ukrainian port Mariupol.
White phosphorus can be used for illumination at night or to create a smokescreen, but when it is deployed as a weapon it causes horrific burns.
A British defence intelligence assessment published by the MOD said white phosphorus had already been used by Russia in the Donetsk region of eastern Ukraine.
The intelligence assessment published by the MOD said: "Russian forces' prior use of phosphorus munitions in the Donetsk Oblast raises the possibility of their future employment in Mariupol as fighting for the city intensifies."
The MOD said Russian forces had continued shelling in the Donetsk and Luhansk areas of eastern Ukraine, but Kyiv's troops had repulsed "several assaults".
However, the MOD warned that Russia's continued reliance on "unguided bombs", decreases their ability to discriminate when targeting and conducting strikes which would "greatly" increase the risk of further civilian casualties.
The UK is set to step up its military support to Ukraine with the £100m defensive aid package announced 24 hours before Boris Johnson's unannounced visit to see Volodymyr Zelensky in Kyiv.
Following the Prime Minister's surprise visit to Ukraine, the UK announced it was to send 120 armoured vehicles to the country.
Cabinet minister George Eustice told Sky News: "We must ensure that Putin cannot prevail on this.
"The Ukrainians have been incredibly brave. I think President Zelensky has led his people well and we've all been humbled really by the acts of bravery we've seen, but also deeply shocked about some of those terrible scenes that we're seeing coming out now from the cities from which Russia's withdrawn."
Watch: UK to send more anti-aircraft and anti-tank missiles to Ukraine, PM says.
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 – although Russia has claimed the scenes from the aftermath of its occupation were staged.
The Ukrainian President Volodymyr Zelensky used his nightly address to warn Ukrainians to brace for a Russian onslaught in the country's east.
"Russian troops will move to even larger operations in the east of our state," he said.
"They can use even more missiles against us, even more air bombs. But we are preparing for their actions.
"We will respond. We will be even more active in providing Ukraine with weapons," the Ukrainian president added.
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