The White House on Tuesday accused Russia of planning to annex more areas of eastern Ukraine through the same "annexation playbook" it used when it seized control of Crimea in 2014.
National Security Council spokesman John Kirby said Moscow was preparing to install proxy officials, establish the rouble as the default currency and force residents to apply for citizenship.
"Russia is laying the groundwork to annex Ukrainian territory that it controls in direct violation of Ukraine sovereignty," Mr Kirby said. "We know their next moves."
He said Russia's actions were a "gross violation of the UN charter".
"We will not allow it to go unchallenged or unpunished," Mr Kirby said,.
He said the American response would include further sanctions imposed on Russia.
Mr Kirby also said that more US military aid for Ukraine would be announced this week as part of another security package, including more high-mobility artillery rocket systems.
Russia invaded Ukraine on February 24 in what it calls a "special military operation".
Mr Kirby said Moscow's annexation plans include Russian holding regional referendums on joining Russia, which would be used as "basis to try to claim annexation of sovereign Ukrainian territory."
Next steps involve introducing branches of Russian banks and using the rouble as its default currency, taking control of broadcasting towers and controlling internet access, and imposing Russian citizenship on locals.
"We will never recognise any purportedly annexed territories belonging to Russia," Mr Kirby said.
He said that the White House believed Ukraine would fight these annexation attempts and that its military would try to retake the territory.
"They absolutely have a right to go on the counter-offensive inside their own country against Russian forces," Mr Kirby said.
He said Washington was hoping for a UN-led agreement between Turkey, Ukraine and Russia on lifting a Black Sea blockade to ensure the global export of grain.
On a reported sale of armed drones from Iran to Russia, he said that the US was "watching this closely and we're taking it seriously".
Ukraine's first lady, Olena Zelenska, was welcomed by US President Joe Biden and his wife Jill at the White House as part of her trip this week, which included a meeting with Secretary of State Antony Blinken.