President Vladimir Putin on Wednesday signed a decree making it easier for people living in eastern Ukraine's separatist territories to obtain a Russian passport.
The decree, published on the Kremlin website, is aimed at residents of the unrecognised Donetsk and Lugansk republics that broke away from Kiev in 2014 and are governed by Moscow-backed rebels.
People living in the rebel republics will be entitled to receive a Russian passport within three months of applying for one, it said.
The move "aims to protect the rights and freedoms of a person and a citizen and is guided by generally accepted principles and norms of international law", the decree said.
The conflict between the Ukrainian government and breakaway rebels began after Moscow annexed Kiev's Crimea peninsula in 2014. The war has claimed some 13,000 lives.
Ukraine elected a new president, comedian Volodymyr Zelensky, last weekend.
He has pledged to "reboot" peace talks with the separatists that also involve Russia and the West.
Ukraine's foreign minister Pavlo Klimkin urged residents in eastern Ukraine not to take Russian passports.
"Russia has deprived you of the present, and now it is trespassing on your future," he wrote on Twitter.
He called the decision "the continuation of aggression and interference in our internal affairs."
Iryna Gerashchenko, the deputy speaker of the Ukrainian parliament, said Putin "always uses moments of uncertainty and transition periods to plunge another knife into Ukraine's back."
She said the move is a "gross violation" of peace agreements signed by the leaders of Russia, Ukraine, France and Germany in Minsk in 2015.
These aimed to put an end to the conflict but fighting has continued with no clear end in sight.