Serbia's president said on Wednesday his country would pay each citizen who gets a Covid jab before the end of May, in what could be the world's first cash-for-jabs scheme.

The Balkans country bought millions of doses -- from Western firms as well as China and Russia -- and briefly became a regional vaccine hub when it offered foreigners the chance to be inoculated.

However, after 1.5 million of Serbia's seven million population received two vaccine doses, with officials regarding 1.3 million fully immunised, the drive has started to stall.

-All those... who received the vaccine by May 31 will get 3,000 dinars (25 euros, $30)- President Aleksandar Vucic told local media, adding that he expected three million to be vaccinated by the end of the month.

Vučić said the country wanted to "reward people who showed responsibility".

But he added that public employees who did not receive a vaccine would not get paid leave if they contracted the virus.

Economy expert and editor of Nova Ekonomija magazine Biljana Stepanović said that Vučić is "acting like Santa Claus" with taxpayer money, and added it was "illegal" to deprive workers paid leave.

Serbia is hoping to further boost its rollout on Thursday by offering vaccines in shopping centres and rewarding volunteers with vouchers.

-I have not encountered in medical literature anyone being paid for Covid-19 vaccinations- Serbian epidemiologist Zoran Radovanovic told AFP.

-So we may be the first not only in Europe, but also the world.-

However, he warned that stimulating vaccination drives with money could be a double-edged sword.

-It will push some of those who live in extreme poverty to get vaccinated, but it will produce resistance with some of the others- said Radovanović.

-The logic is this: if the state is paying me to do something it is telling me is for my own good, then something is fishy.-

Some companies have offered employees incentives to get the Covid jab, and poorer countries in Asia and Africa are known to have offered rewards to those getting vaccinated in other campaigns -- but not since the pandemic took hold.