Vladimir Putin has swiftly moved to test Ukraine’s new president-elect, signing a law that could allow millions of Ukrainians living in breakaway regions of south-east Ukraine to receive Russian passports.

The provocative decision came just three days after the 41-year-old Volodymyr Zelenskiy, a comedian with no political experience, won a landslide victory in presidential elections. The Kremlin has not yet congratulated Zelenskiy or officially recognised the election result.

In campaigning, Zelenskiy had signalled he could be more amenable to compromise on the separatist regions than the outgoing president, Petro Poroshenko, and he campaigned in part on Ukrainians’ exhaustion with the war.

Now, Russia’s challenge could force him to take a harder line. His team released a statement on Wednesday calling Russia’s move another sign that Russia was “an aggressor state that is waging war against Ukraine”.

More than 13,000 people have died in eastern Ukraine since fighting broke out in 2014 between the government and separatists armed and financed by Russia. The offer of Russian passports will be seen in Kyiv as an attempt to continue the integration of the territories of the Donetsk and Luhansk regions into Russia, as well as to pressure the new president to come to the negotiating table.

“It’s raising the stakes. This is a serious message for Ukraine as well as the west,” said Balázs Jarábik of the Carnegie Endowment for International Peace. “This would make the status of the territories more and more complicated, and the idea is to force Zelenskiy to start negotiations as soon as possible.”

The US embassy in Kyiv called the Kremlin decree “absurd and destablising”, whilePavlo Klimkin, the Ukrainian foreign minister, tweeted: “I call on Ukrainian citizens in Russian-occupied territories not to accept Russian passports. Russia has deprived you of the present, but now it encroaches on your future.”

The text of the law, which appeared on the Kremlin’s website on Wednesday, granted Ukrainians living in parts of the Donetsk and Luhansk regions “the right to request citizenship of the Russian Federation” as a humanitarian measure. Requests for citizenship should be considered within three months, the bill said.

Vladislav Surkov, a top Kremlin official on Ukraine, hailed the decision. “Ukraine refuses to accept these citizens as their own, installing an economic blockade, not allowing them to vote, using military force against them. Having received a passport, these people will feel more secure and free,” he told Russian news agencies.

Ukrainian separatist leaders estimate 3.6 million people still live in the regions of Donetsk and Luhansk no longer under Kyiv’s control, although the real number is likely to be smaller.

The timing of the decree, just after Zelenskiy’s victory, may make it politically harder for the incoming president to seek compromise. Prior to the election, Zelenskiy’s chief of staff Ivan Bakanov signalled during meetings in Washington that the Zelenskiy team would be amenable to negotiating directly with separatist authorities, according to a source briefed on the contents of the discussions. Western diplomats have cautioned the Zelenskiy team against such a course, as it would play into Russia’s narrative of an internal conflict.

Zelenskiy has also called for a Russian-language television channel to target those people living in the separatist areas in a hearts-and-minds operation.

In contrast, Poroshenko campaigned on a patriotic line emphasising his support for the army and the country’s defence against Russia. On Thursday, Ukraine’s parliament will vote on a controversial language law that some of the country’s Russian speakers have said could curtail their rights.

Alexei Chesnakov, an adviser to Surkov, said the Kremlin had waited for the result of the presidential election on Sunday before issuing the decree.

“The Kremlin didn’t do this earlier in order not to give additional pretexts for Poroshenko and the country’s radicals to use it during the campaign,” he said.

Unlike Crimea, which Russia annexed swiftly in 2014, Donetsk and Luhansk regions have been left in legal limbo in the past years. Despite official denials that it controls the territory, Moscow has tightened its control since the peak of the conflict in 2014 and 2015. Alexander Khodakovsky, a former separatist leader who was pushed out of office and is now based in Russia, estimates that 60% of the budget of the so-called Donetsk People’s Republic currently comes from Russian subsidies.

A number of separatist leaders have met untimely deaths in bombings and other attacks over the past three years, and been replaced with more pliant civilian figures who answer directly to Moscow. Officially, the assassinations are blamed on Ukrainian diversionary groups.

The decision to hand out passports will be seen as a game-changer, and mirrors policies in the breakaway republics of Abkhazia and South Ossetia. Russia justified its military intervention in a 2008 war with Georgia, and subsequent recognition of the territories as independent states, in part because of the large number of Russian passport holders in the region.