Unrelated cases of polio in South Sudan and Madagascar have been confirmed by the World Health Organization, which is blaming low vaccination coverage.
The UN health agency said two cases of so-called type two vaccine-derived poliovirus had been confirmed in South Sudan in camps for people displaced by the country's civil war.
The crippling disease, which usually impacts young children, had been found in two patients in Unity State hit with acute flaccid paralysis (AFP), WHO said.
Vaccine-derived polio infections are in rare cases caused by one type of polio vaccine, which contains small amounts of weakened but live polio virus.
Oral polio vaccine (OPV) replicates in the gut and can then be passed to others through faecal-contaminated water, thus imperilling unvaccinated children.
WHO has recommended that OPV be phased out worldwide and replaced by the inactivated polio vaccine (IPV).
The agency pointed out that as many as a third of children in restive Unity State remained under-vaccinated against polio, increasing the risk.
WHO said there was little risk of a wider spread, stressing that vaccination campaigns could easily be carried out in the internal displacement camps.
"However, the risk of international spread would increase if other areas are infected," it said.
Vaccine-derived polio virus, this time type one, had also been confirmed in Madagascar, in one person with AFP, and in three people showing no symptoms, WHO said.
The agency said around a quarter of children on the island were under-immunised against polio, in a country which has been hit with two similar events since 2001, each resulting in five cases.
"Concerted outbreak response each time rapidly stopped those events," WHO pointed out, stressing though that the repeated emergence of the cases "underscores the risk of these events occurring in populations which are not fully immunised."