President Vladimir Putin on Wednesday fast-tracked citizenship for residents of two regions of Ukraine, prompting protests from Kyiv that the move violated its sovereignty.
Putin signed a decree affecting residents of the southern region of Kherson, which is under the full control of Russian troops, and the southeastern region of Zaporizhzhia which is partially controlled by Moscow.
Moscow and pro-Moscow officials have said both regions could become part of Russia.
"The simplified system will allow all of us to clearly see that Russia is here not just for a long time but forever," the Moscow-appointed deputy leader of Kherson region, Kirill Stremousov, told Russia's RIA Novosti state news agency.
"We are very grateful to Russian President Vladimir Putin for all he is doing for us, for protecting Russian people in historically Russian lands that have now been liberated," he added.
The new authorities want to help those wishing to "join the big family of Russia", he said.
The Ukrainian foreign ministry swiftly protested against the "illegal issuing of passports".
The move "is a flagrant violation of Ukraine's sovereignty and territorial integrity, as well as norms and principles of international humanitarian law", it said in a statement.
In Washington, State Department spokesman Ned Price voiced concern that the plan was part of "Russia's attempt to subjugate the people of Ukraine -- to impose their will by force".
"That is something that we would forcefully reject," Price told reporters.
The official order published Wednesday came on the heels of a 2019 decree that allowed the same fast-track procedure for residents of the self-proclaimed Donetsk and Lugansk People's Republics, eastern Ukraine's breakaway regions.
Applicants are not required to have lived in Russia, do not need to provide evidence of sufficient funds or pass a Russian language test.
Applications will be processed within three months and the Kherson region has already begun work on launching centres to issue Russian passports, Stremousov said.
Several hundred thousand residents of Ukraine's Donetsk and Lugansk regions have already received Russian passports.
On Monday, the authorities in Kherson introduced the ruble as the official currency alongside the Ukrainian hryvnia. On Wednesday, officials installed by Moscow announced the same measure in parts of the region of Zaporizhzhia.