Somalia is suffering an "explosive" outbreak of polio and now has more cases _ 105 _ than all other countries in the world combined, a World Health Organization official said Friday. The outbreak is complicated by the fact health workers have limited access to south-central Somalia, controlled by al-Qaida-linked militants.

Health officials are responding with vaccination campaigns that have reached 4 million people since the outbreak began in May, but those health officials have limited access to about 600,000 children who live in areas of Somalia controlled by the armed Islamist group al-Shabab.

In addition to the 105 cases, another 10 cases have been confirmed just across the border in Kenya in what is essentially the same outbreak.

"The first thing to say is it's very worrying because it's an explosive outbreak and of course polio is a disease that is slated for eradication," said Oliver Rosenbauer, a spokesman for the global polio eradication initiative at the World Health Organization in Geneva. "In fact we're seeing more cases in this area this year than in the three endemic countries worldwide."

Polio is considered eradicated globally except for those three endemic countries: Afghanistan, Nigeria and Pakistan. In a bit of good news, though, Rosenbauer said polio numbers are down in those three countries. Globally there have been 181 cases of polio this year, including those in Somalia and Kenya. Somalia was removed from the list of endemic polio countries in 2001, but this year's outbreak is the second since then.

In the al-Shabab area of south-central Somalia, disease surveillance is functioning and cases have been confirmed from the region, but health officials are likely not able to detect all the cases.

"We have a much less sense of how the vaccination campaigns are happening," Rosenbauer said. "We have less ability to monitor and see how the campaigns are going. What we do know based on the vaccination status of suspected cases is that the vaccination status is lower there than in the accessible areas. For sure it's a complicating factor."

Mohamud Yasin, a retired doctor who has treated polio throughout his career, said: "It's indeed worrying because this comes at a time when the country is still hugely affected by the raging fighting which prevents volunteers from accessing people in need of vaccines. It may take time before we can confidently say we have has a universal coverage of the immunization."

At the Global Vaccine Summit in the United Arab Emirates in April, Bill Gates helped unveil a six-year plan to eradicate polio. The eradication effort will cost $5.5 billion, three-quarters of which has already been pledged, including $1.8 billion from the Bill and Melinda Gates Foundation.

The outbreak in Somalia does not set back the six-year plan, said Rosenbauer, because unpredictable and intermittent outbreaks were programmed into the timeline.

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Associated Press reporter Abdi Guled in Mogadishu, Somalia contributed to this report.