Vladimir Putin's controversial move to offer fast-tracked Russian passports to residents of separatist-controlled eastern Ukraine sparked outcry in Kyiv and other European capitals.

Russia's president was accused of attacking Ukraine's sovereignty and putting in peril a ceasefire in the region.

But little attention was paid to how much Putin's offer could cost.

There were 681,000 pensioners in the so-called Donetsk People's Republic, as of November 2017, and 438,000 in Luhansk People's Republic, according to April 2018 figures.

If all 1.1 million of them moved or came under the control of Russia and claim a minimum pension of 8,000 roubles (€110) that would represent a major outlay for Moscow: it would cost in the region of 110 billion roubles (€1.5m).

Putin's testing timing

"Russian passports will ease the life of people in Donbass," said Russian demography expert Olga Chudinovskikh.

"It will be easier for them to get a job in Russia, they will be able to receive social benefits and pensions.

"But it is not only a matter of increasing spending on social benefits but also of probable legalisation of people with criminal backgrounds in the so-called republics. In addition, the mass distribution devalues the Russian passport."

The controversial Russian decree came just three days after Volodymyr Zelenskiy, a 41-year-old comedian, won a landslide victory in presidential elections in Ukraine.

"The decree was released at the strategic moment when the Russian-speaking population of the Donetsk and Luhansk regions will be in the centre of attention of the newly elected president of Ukraine," said Olga Gulina, a senior researcher of the Berlin-based RUSMPI UG Institute on Migration Policy.

"Strategically, having signed the decree, the President of Russia, put a question to the newly-elected President of Ukraine on how important the Eastern regions of Ukraine are for his rule."

How should Ukraine react?

Grigoriy Perepelitsa, the director of the Foreign Policy Institute of the Diplomatic Academy of Ukraine said the decree might have very dramatic consequences for Ukraine.

"The decree signed by Putin gives Russia a legal ground not only to openly bring in the Russian troops to protect the Russian citizens but also to occupy the territory of Eastern Ukraine," he said. "This is practically an occupation of the Ukrainian territory.

"The only thing Ukraine can do now is to entrench the status of governmentally non-controlled territories as occupied territories, which wouldn't allow Russia to grant citizenship and [carry out] military recruitment.

"We have to identify Russia as an aggressor and break all ties with the country."

Ukraine is said to be working on a new law that might lead to people losing their Ukrainian citizenship if they apply for a Russian passport.

But what else should Kyiv do?

Oleksiy Matsuka, the head of the Donetsk Information Institute, thinks Ukrainian authorities lack clarity over the future of Donbass.

"We need clarity," he said. "And first of all, people living in Donbass need certainty about Ukrainian intentions.

"For the third year in a row, people have been watching the Ukrainian government, which has not been fully providing pensions to Donetsk and Luhansk retirees.

"The rhetoric of the officials switches from imperfect payment mechanism to hints that pensions and social benefits at the occupied territory are the responsibility of the aggressor country."

How did Russians react?

Inside Russia, Putin's decree was widely supported. Margarita Simonyan, the editor-in-chief of the state-owned Russia Today was among them.

"It finally happened!!! I'm screaming and cheering! Putin signed a decree on Russian citizenship for the people of Donbass!" Simonyan wrote on social media.

Still, some of the Russian experts are sceptical about whether escalating the confrontation with Ukraine is beneficial for Russia.

"It is Russia that turns out to be in a weak position compared to Ukraine because literally on each legal issue, whichever you take, it [Russia] turns out to be wrong. And that's why it is Russia, not Ukraine, that is in the global isolation and under the sanctions," wrote Russian sociologist Alexey Roshchin on his Facebook page.

"[The] Russian Federation is simply dragged deeper and deeper into 'confusion', into problems that cannot be resolved. We dragged ourselves into the new sanctions, did not really obtain anything, and got even closer to the status of an outcast."