The disruption to an emergency polio vaccination programme in northern Nigeria threatens the entire region, United Nations officials have warned.
Most parts of Nigeria have embraced the vaccine programme
Nigeria has the world's highest number of cases, but three northern states halted the drive after an Islamic leader declared the vaccines unsafe.
The UN warns the virus now threatens areas that had been polio-free.
Unicef's Dr Abdullah Pinorgah said high-level political intervention may be needed to get the programme resumed.
The medical chief for the UN children's fund told the BBC the vaccine was entirely safe and was used worldwide.
"Immunisation is associated with a lot of religious, cultural issues, and we've had this problem in other parts of the world, misconceptions about what is contained in the vaccine," he said.
The WHO launched its campaign last week in a bid to immunise more than 15 million children in west and central Africa against polio.
The new danger stems from Nigeria, which accounts for nearly half of all polio cases in the world.
But three predominantly Muslim states in northern Nigeria - Kano, Kaduna and Zamfara, have delayed or refused permission following opposition from influential Islamic leaders who allege the oral vaccination is unsafe and part of a secret US plan against Africa.
This Nigerian child is now protected
Datti Ahmed, who is both a medical doctor and the president of Nigeria's Supreme Council for Sharia law, said they needed to check reports that the vaccine was contaminated.
"There were strong reasons to believe that the polio immunisation vaccine was contaminated with anti-fertility drugs, contaminated with certain virus that cause HIV/Aids, contaminated with Simian virus that are likely to cause cancers," he told the BBC World Service's Newshour programme.
Rumours have been circulating in northern Nigeria that the vaccine is part of a US plot to limit the country's population by spreading Aids and rendering people infertile.
When asked about such reports, Dr Ahmed said they might sound insane but they had to be checked out.
The head of the WHO's polio campaign, David Heymann, told the BBC that the vaccines used in Nigeria are no different from those used to combat polio everywhere else in the world.
With the take-up of vaccines so low in northern Nigeria, polio was taking firm hold, Mr Heymann warned.
"They are spreading it... into the central Nigerian states and even into Lagos, and in addition to five neighbouring countries, which are polio-free."
Nigeria's neighbours are at risk
These countries were now, at great cost, having to build a wall of immunity around the imported cases, he said.
Hundreds of thousands of health workers and volunteers have been mobilised across the region to treat millions of children.
Already new polio cases have identified in Niger, Burkina Faso, Togo, Ghana and Chad.
The latest crisis threatens to undermine a global initiative to eradicate the crippling disease by the end of 2004.
Poliomyelitis is an acute viral infection which mainly affects children and can be spread by simple physical contact.
It causes permanent paralysis and other forms of physical disability in many of its victims.