Russia is laying the groundwork to annex Moscow-controlled Ukrainian territory amid the war, a White House official said Tuesday based on U.S. intelligence, warning that any such attempt would be met with more sanctions from the United States and its allies.

"The Russian government is reviewing detailed plans to purportedly annex a number of regions in Ukraine" including Kherson and Zaporizhzhia in the south and Donetsk and Luhansk in the east, John Kirby, National Security Council spokesman, told a press conference.

He also said Russia is beginning to roll out a version of what can be called an "annexation playbook," using tactics very similar to those seen in its annexation of Crimea from Ukraine in 2014.

U.S. National Security Council spokesman John Kirby speaks during the daily press briefing at the White House July 19, 2022 in Washington. (Getty/Kyodo)

Russia is already installing what the United States views as illegitimate proxy officials in the areas of Ukraine that are under its control, and these officials are expected to arrange "sham referenda" on joining Russia, which Moscow will use as a basis to claim Ukrainian territory, Kirby explained.

But he warned that the United States plans to respond "swiftly and severely" and "in lockstep with our allies and partners" should Russia proceed with the annexation plans.

"Russia will face additional sanctions and become even more of a global pariah than it is now," Kirby said. "We will never recognize any purportedly annexed territories belonging to Russia."

The four regions cited as potential annexation areas are contiguous and occupying them would connect Russia with the already seized Crimean Peninsula.

Russia is seeking to set up branches of Russian banks to establish the ruble as the default currency in the areas. In Kherson, Russia has taken control of broadcasting towers, establishing loyalist security forces and forcing residents to apply for Russian citizenship and issuing Russian passports, Kirby said.

While the Kremlin has not disclosed a timeline for referendums, Kirby said Russian proxies in the controlled territories have claimed they will take place later this year, possibly in conjunction with Russia's September regional elections.

Russia began its military attack against Ukraine on Feb. 24 after asserting that its security was under threat from the North Atlantic Treaty Organization's eastward expansion and the possibility of Ukraine joining the security alliance.

Since failing to seize the Ukrainian capital of Kyiv in the early stage of the war, Russia has shifted its focus to expanding its control in Donetsk and Luhansk -- home to Russia-backed separatists who have been fighting Ukrainian forces since 2014 -- and taking control of the southern area.