The United States on Tuesday accused Russia of moving ahead with plans to annex additional Ukrainian territory.
"Russia is laying the groundwork to annex Ukrainian territory that it controls in direct violation of Ukraine's sovereignty," National Security Council spokesman John Kirby told reporters, according to the AFP news agency.
"Russia is beginning to roll out a version of what you could call an annexation playbook, very similar to the one we saw in 2014," when it invaded and eventually annexed Crimea, Kirby said.
"Already, Russia is installing illegitimate proxy officials in the areas of Ukraine that are under its control," he added.
Kirby also said Russia had plans to organize "sham referenda" in the areas it has seized, possibly as early as September.
"Annexation by force will be a gross violation of the UN Charter and we will not allow it to go unchallenged or unpunished," he said, adding was "exposing" the Russian plans "so the world knows that any purported annexation is premeditated, illegal and illegitimate."
The areas targeted for annexation, continued Kirby, include Kherson, Zaporizhzhia, all of Donetsk and Luhansk.
Shortly before he launched the invasion of Ukraine this past February, Russian President Vladimir Putin recognized the two separatist regions in Ukraine of Donetsk and Luhansk as independent and later announced he would deploy "peacekeeping" forces there.
He then said that rebel leaders in eastern Ukraine had asked Russia for military assistance to help fend off Ukrainian "aggression", fueling fears that the rebel request amounted to a pretext for war.