Russia on Wednesday made it easier for people living in eastern Ukraine's separatist territories to obtain Russian passports, drawing swift condemnation from Kiev only three days after it elected a new president.
President Vladimir Putin signed the decree 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 separatist regions will now be entitled to receive a Russian passport within three months of applying for one.
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's president-elect Volodymyr Zelensky, a comedian who won Kiev's weekend vote, called for more international sanctions against Russia in response to the move.
"This is another clear confirmation for the world of Russia's real role as an aggressor state, waging war against Ukraine," his press service said in a statement.
Due to be inaugurated by early June, Zelensky has pledged to "reboot" peace talks with the separatists that also involve Russia and the West.
Ukraine's outgoing President Petro Poroshenko also slammed the move, calling it "unprecedented interference."
Other officials in Kiev said the timing was aimed at destabilising Ukraine during a period of transition.
- 'Knife in Ukraine's back' -
Putin said the new law was "purely a humanitarian issue."
"We have no desire to create problems for the new Ukrainian authorities," he said during a meeting with lawmakers in Saint Petersburg.
The Russian leader added that people living in the rebel republics are "completely deprived of any civil rights".
"They cannot move normally, cannot realise their most elementary needs," he said.
According to a copy of the decree published on the Kremlin website, the law aimed to "protect rights" and was guided by "the norms of international law".
But Ukraine's foreign minister Pavlo Klimkin called the decision "the continuation of aggression and interference in our internal affairs."
He 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.
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 was 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.
The US embassy in Kiev also condemned "Russia's recent absurd and destabilising decree" in a tweet on Wednesday.
Zelensky, whose only previous political experience so far has been limited to playing the role of president in a TV show, won Sunday's vote with a landslide victory.
He has said he wants to "renew relations" with the residents of eastern Ukraine, saying "they are also Ukrainians."
In his statement on Wednesday, he said one of the priorities of his presidency would be to find solutions to the country's internal refugee problem and expanding ways to maintain links with people living in separatist territory.
The actor had previously announced plans to start a "powerful information war to end the conflict."
People living in eastern Ukraine mainly have access to Russian television channels.
On the campaign trail, Russian-speaking Zelensky criticised some of Kiev's anti-Moscow policies while saying he would keep Ukraine on a pro-Western course.
He has also said he would not resort to force to take back Crimea.
"The newly elected president is being tested for his strength," Ukrainian lawmaker Sergiy Vysotsky wrote on Facebook.
Russia's reaction to his win has been mixed, with the Kremlin refusing to congratulate him but others seeing an opportunity to improve ties.