Russia plans to annex more Ukrainian territory using a similar “playbook” to its takeover of Crimea, the US says.
Citing US intelligence, National Security Council spokesman John Kirby said Russia is already laying the groundwork for annexation.
Occupied regions of Ukraine could hold “sham” referenda on joining Russia as soon as September, he said.
Russia annexed Crimea from Ukraine in 2014 after a referendum which was widely viewed as illegitimate.
“We want to make it plain to the American people,” Mr Kirby told reporters. “Nobody is fooled by it. [Russian President Vladimir Putin] is dusting off the playbook from 2014.”
He accused Russia of installing illegitimate pro-Russian officials to run occupied regions of Ukraine, with the aim of organising referenda on becoming part of Russia.
The results of the votes would be used by Russia “to try to claim annexation of sovereign Ukrainian territory”, Mr Kirby said.
Russia has already installed its own regional and local officials in the parts of Ukraine it has occupied.
Crimea was annexed by Russia in 2014 after a hastily-organised referendum – viewed as illegal by the international community, in which voters chose to join Russia.
Many supporters of Kyiv boycotted the vote and the campaign was neither free nor fair.
Similar votes held in other parts of Ukraine would almost certainly see a similar situation, with any opposition to joining Russia largely supressed.
Mr Kirby said he was “exposing” the Russian plans “so the world knows that any purported annexation is premeditated, illegal and illegitimate”, and promised there would be a quick response from the US and its allies.
The areas targeted for annexation include Kherson, Zaporizhzhia, Donetsk and Luhansk, he said.