Moscow - Ukrainian President Petro Poroshenko on Wednesday accused Moscow of crossing a "red line" by signing a decree to ease the process of obtaining Russian citizenship in eastern Ukraine.
"It's a fact that this is about the Kremlin preparing the next point of aggression against our country: the annexation of Ukraine's Donbass or the creation of a Russian enclave in Ukraine," he said.
Poroshenko, who lost the second round of Ukraine's presidential elections on Sunday, called on the international community to prevent "the worst scenario" and tighten sanction on Russia over the move.
The team of President-elect Volodymyr Zelensky - a comedian who has never held a political post - also criticised the Kremlin's move.
"This action is another obvious confirmation for the international community of Russia's real role as an aggressor state," the team said.
The Kremlin said Wednesday that President Vladimir Putin had signed a decree making it easier for people living in separatist-controlled regions of eastern Ukraine to obtain Russian passports.
Residents with a permanent residence in "individual counties" in the Donetsk and Luhansk regions can become Russian citizens through a "simplified process," according to the Kremlin. The statement refers to shortening the time needed to examine the necessary documents.
Moscow has already been recognizing documents such as birth certificates or vehicle registrations from the separatist regions' authorities.
The Ukrainian ambassador to the UN, Volodymyr Yelchenko, said on Twitter that his country would be asking the UN Security Council to prohibit Russia from enacting the decree in the ex-Soviet republic.
Ukrainian Foreign Minister Pavlo Klimkin urged Ukrainian citizens in the Russian-occupied regions not to apply for Russian passports.
"Russia has deprived you of today and is now grabbing at your future," wrote Klimkin on Twitter after the announcement.
According to Russia's Interfax news agency, the reaction in Ukraine prompted Putin to say that he did not want problems with the new government, but instead was concerned about human rights in the area.
According to UN estimates, about 13,000 people have been killed in the conflict near the Russian border between pro-Russian separatists and the Ukrainian military that erupted after the ouster of pro-Russian president Viktor Yanukovych in 2014.dpa