(Adds details of vaccination campaign)
JUBA, Sudan, Oct 27 (Reuters) - A mass polio vaccination began on Monday across southern Sudan, where at least six cases of the debilitating disease have been reported since March, U.N. health officials said.
Until March, no cases had been recorded in South Sudan since 2004, when an outbreak infected 12 people in the region, said Afework Assefa, World Health Organisation (WHO) team leader for South Sudan's polio programme.
"We hope this campaign will control the outbreak," he said. "Even one case is worrying, and in so many different states (regions) it is concerning."
Health workers will try to immunise 2.8 million children below the age of five under the health campaign.
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.
Government officials estimate that only 25 percent of the population can access healthcare easily. Assefa said few have access to routine immunisation for their children.
Without regular national immunisation, the highly infectious virus would probably spread across the south, he said.
Neighbouring countries such as Kenya and Uganda, which have trade links with South Sudan, have been warned about the reappearance of the virus.
Experts believe the outbreak came from Ethiopia, Assefa said. Uganda is conducting similar vaccination campaigns.
Polio spreads through contaminated food and water and thrives in areas with poor sanitation.
Polio has been eradicated from most of the globe, but can still be found in areas including northern Nigeria, northern India and along the Pakistan-Afghanistan border.
In 2007, 1,310 cases were reported worldwide, according to the U.S. Centers for Disease Control and Prevention. (Reporting by Skye Wheeler; Editing by Jon Boyle)