An emergency campaign has begun to try to immunise more than 15 million children in west and central Africa in just three days against a new threat of polio.
Health officials have urged Nigeria to do more to eradicate polio
Hundreds of thousands of health workers and volunteers are aiming to treat all children in Benin, Burkina Faso, Ghana, Niger and Togo following a fresh outbreak in neighbouring Nigeria.
Nearly half of all polio cases in the world are in Nigeria - virtually all of them in the north of the country - and the aim of the current campaign is to halt the spread of the virus across Nigeria's borders.
"It would not have been necessary to have this firewall around Nigeria last year, but because the virus has spread to a number of neighbouring countries, we need to launch a campaign to cover 15m children at a cost of $10m," Bruce Aylward, the coordinator of the World Health Organisation's (WHO) Polio Eradication Programme told the BBC.
He says the reason for the spread was the failure to reach and immunise all the children in the area over the past two years.
50% polio cases in Nigeria
15m children to be immunised
Goal - polio-free-world by 2004
"We face a grave public health threat and our goal of a polio-free world is in jeopardy," WHO polio specialist David Heymann warned.
The latest crisis threatens to undermine a global initiative to eradicate the crippling disease by the end of 2004.
As well as this $10 million project, vaccination campaigns are also planned for Cameroon and Chad in the next few weeks.
WHO also expressed concern that polio-infected states in northern Nigeria, particularly around the state of Kano, had re-infected other areas of the country, including Lagos.
It also said a "difficult environment" compromised the quality of immunisation campaigns and aided the spread of rumours about the safety of the oral polio vaccine.
One Nigerian state is reported to have suspended its immunisation programme, with concerns about the vaccine being expressed in several states.
United Nations Children's Fund (Unicef) Executive Director Carol Bellamy said Nigeria, as the most populous country in the region, had a responsibility to its neighbours to ensure the disease was eradicated.
"We simply cannot afford to see these isolated viruses again paralysing children in areas which had previously been polio-free," she said.
"That is why this massive campaign is critical."
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.
In 2002, WHO said 1,919 cases were reported, compared to 483 in 2001, an increase partly attributed to a rise in polio cases in Nigeria.