In its latest update on Friday, the UK's Ministry of Defence announced that Russian forces have fully withdrawn from northern Ukraine to Belarus and Russia.
The Ministry added that "at least some of these forces will transferred to east Ukraine to fight in Donbas" which comprises the separatist regions of Donetsk and Luhansk.
It also said that many of these of these forces will need "significant replenishment before being ready to deploy further east".
"Any mass redeployment from the north likely to take at least a week minimum," the Ministry said in the update posted on Twitter.
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It added that Russian shelling of cities in Ukraine's "east and south continues and Russian forces have advanced further south from the strategically important city of Izyum" which remains under Moscow's control.
Russia's main effort now is eastern Ukraine, after withdrawing its forces from the area around Kiev in the north, the BBC said in a report.
