The World Health Organisation says two young children have been paralysed in the first polio outbreak in Europe for five years.
Both cases are in south-western Ukraine, near the border with Romania, Hungary, Slovakia and Poland.
Experts say the outbreak must be contained, as the risk of spreading is high.
“For each child with paralysis, there are about 198-199 children that get infected by the virus but who don’t get paralysed.
These children play with others who also have their own network of contacts.” said Dorit Nitzan from the World Health Organisation.
Ukraine was particularly at risk due to inadequate uptake of vaccination.
Only 50% of the country’s children were fully immunised in 2014..
“We need to do an immediate campaign in the affected area next week. Then we will be ready to immunise all children in Ukraine.” said Dragoslav Popovic, the UNICEF representative in Ukraine.
Experts say outbreak like this are rare but do emerge in areas of inadequate immunisation.
They recommend everyone visiting the region should be fully vaccinated.
Residents and those staying for more than a month should get an extra dose of vaccine.