JUBA, Sudan (Reuters) - Medical authorities plan to intensify a mass immunisation campaign in southern Sudan, where earlier efforts had failed to stem a polio outbreak that has so far this year infected 15 people, officials said on Friday.

Anthony Laku Steven, the Health Ministry's Director of Community-based Healthcare, said very few babies, especially in rural areas, receive the polio vaccine at birth.

"We are very worried. South Sudan was supposed to be declared free of polio," he said. "Even one case is an emergency. It spreads very fast to other children."

The semi-autonomous south of Sudan has been struggling to improve its health sector since a 2005 peace deal ended a decades-long civil war with the north.

Until March this year, south Sudan was considered polio-free, with no new cases since 2004 when 12 people were infected in an outbreak. Children under three are most vulnerable to the disease that can cause irreversible paralysis.

Officials held two national immunisation days in October and November - in which 2.8 million children across the south were targeted - to try to interrupt the virus that spreads through contaminated food and water.

But heavy rains that shut down roads and air strips meant fewer children were reached than hoped and it continued to spread including into new areas, said Afework Assefa, head of the south's World Health Organisation polio programme.

Six cases have been confirmed in oil-rich Unity State, Assefa added. Genetic testing suggests the virus was imported from next-door Ethiopia.

Four monthly mass campaigns with rejuvenated local planning are set for next year before the dry season ends in April. "It can be done. We are optimistic that by April we will have interrupted transmission," Assefa said.