Russia has moved from having 40% of its forces inside Ukraine to around 75%, a military academic says.
The 75% figure has also been cited by a senior US defence official.
Dr Jack Watling is a research fellow in land warfare and military sciences at the Royal United Services Institute.
He told BBC Radio 4's Today programme a large body of Russian troops is advancing south from Belarus and starting to set the conditions to be able to conduct an assault into Kyiv.
Asked about the suggestion that Russia has used grad missiles on residential areas in Kharkiv, he describes these weapons as multiple launch rocket systems which fire large numbers of unguided missiles into an area.
He says there's also evidence some of these many have contained cluster munitions.
"It's the fact that they're unguided and you're putting huge amounts of high explosive into densely populated civilian area," he says.
The Ukrainian army is no longer functioning in military formations, but now in fairly fixed defending positions and becoming more and more part of the territorial defence and volunteer force that it is operating alongside, he says.