Tens of thousands of Russians are being sent to eastern Ukraine as part of an offensive planned after 15 February, according to the Ukrainian governor of Luhansk region.
"We are seeing more and more reserves being deployed in our direction," said Serhiy Haidai, who expected a three-pronged Russian advance.
Ukraine has warned repeatedly of an imminent offensive.
But there is widespread scepticism of significant Russian success. 1บาคาร่า
The UK's defence intelligence briefing said Russia's aim was almost certainly to capture parts of Ukraine's eastern Donetsk region that were not already under occupation.
But the UK said "it remains unlikely that Russia can build up the forces needed to substantially affect the outcome of the war within the coming weeks".
Russian Defence Minister Sergei Shoigu said seven Ukrainians towns had been "liberated" in recent weeks and operations were progressing "with success" in two more, Bakhmut and Vuhledar.