Britain's defence ministry said on Saturday that the bulk of Russian forces involved in the advance on Ukraine's capital Kyiv were now 30 km (19 miles) from the city centre.
Britain, which earlier disputed Russian reports that Russian forces had captured the southeastern city of Melitopol, said the Ukrainian military was continuing to put up staunch resistance across the country.
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"Russian forces have continued their advance on Kyiv with the bulk of their forces now 30 km from the centre of the city," the defence ministry said in an intelligence update posted on Twitter.
"Russia has yet to gain control of the airspace over Ukraine greatly reducing the effectiveness of the Russian Air Force. Russian casualties are likely to be heavy and greater than anticipated or acknowledged by the Kremlin," it said.