MOGADISHU, Somalia - Islamic extremist rebels are fighting a campaign in Somalia to administer a polio vaccine, charging that it contains the virus that causes AIDS or could make children sterile, a battle of words that is frustrating health workers.

Al-Shabab, the rebels linked to al-Qaida, have discouraged many parents from getting their children inoculated against polio, a disease that is an incipient problem in this Horn of Africa nation long plagued by armed conflict and disease, according to health workers who spoke to The Associated Press.

The al-Shabab extremists have been pushed out of virtually all of Somalia's cities and face continued military pressure from African Union and government troops. Health workers are gaining access to more children to give the lifesaving polio vaccine. But some mothers and fathers are refusing the inoculation, apparently heeding the advice of the Islamic militants who warn that the vaccination exercise is part of a foreign conspiracy to kill or weaken Somali children.

Vaccination workers who walked door to door in the capital, Mogadishu, were turned away by some parents who often didn't state why they objected to the vaccination. One man told the workers to leave immediately because they were carrying "toxic things."

Al-Shabab militants are spreading rumors against the polio vaccine in communities where they still have some influence, alleging the vaccine can make girls barren and that it is manufactured in Christian countries, said a senior United Nations health worker in Somalia.

Al-Shabab did not respond to questions about the allegations that they are spreading rumors against the vaccination campaign.

"Al-Shabab are paranoid about potential infiltration by spy agencies disguised as humanitarian workers. That's probably a principal reason for discouraging vaccination," said Abdi Aynte, the director of the Somali-based think-tank Heritage Institute for Policy Studies.

Somali government officials say the numbers of parents who reject the immunization campaign are far fewer than those embracing it, but health workers don't want to leave any unvaccinated. They warn that it is important for every child to get the polio vaccine in order to eradicate in Somalia the disease that causes limb paralysis and can be fatal.

"It's a big challenge," said Safiyo Mohamed, a vaccination worker in Mogadishu. She said in rejecting the vaccine some families had brought up the case in Libya where foreign health workers - including five Bulgarian nurses - faced charges of deliberately infecting Libyan children with HIV in 1990s.

The polio vaccine, which is administered orally, is recommended for children aged 10 and under, but some parents are questioning why a 10-year-old needs protection against polio, said Biyod Yasin, a vaccination worker at Mogadishu's Hamarweyne Mother Child Centre.

At a world vaccine summit in Abu Dhabi last month, global health leaders and philanthropists presented a six-year, $5.5 billion plan that they hope will eradicate all types of polio disease.

The World Health Organization says Somalia is experiencing an outbreak of polio. WHO reported earlier this month that a 4-year-old Somali girl near the Dadaab refugee camp in Kenya and two of her contacts tested positive for polio, the first confirmed cases in Kenya since July 2011. Four polio cases have been confirmed in Somalia so far this year, including two in Mogadishu. A vaccination campaign targeting 440,000 children began here on May 14, and a second round of the campaign that started Tuesday aim to vaccinate 644,000 children under the age of 10.

Health workers say they vaccinate hundreds each week despite the resistance. That the health workers can move around at all is a stunning turnaround in Somalia, which when al-Shabab radicals ran most of the country refused western medical care and food aid to desperate Somalis.