A senior United Nations official has condemned Nigerian states that have refused to take part in a mass immunisation drive against polio.
Kano officials are still not happy with the vaccine
"It is unforgivable to allow still more children to be paralysed because of... baseless rumours," said UN Children's Fund head Carol Bellamy.
Some Muslim clerics say the vaccine is a western plot to make women infertile.
The governor of one of the boycotting states has said the spread of the disease was "a lesser evil".
"It is a lesser of two evils to sacrifice two, three, four, five, even 10 children (to polio) than allow hundreds of thousands or possibly millions of girl-children likely to be rendered infertile," Kano state Governor Ibrahim Shekarau told the AP news agency.
The mainly Muslim states of Zamfara and Kano have refused to take part in this week's vaccination campaign.
Two other states, Niger and Bauchi, which had signalled they would not take part, have now joined the drive.
Ms Bellamy insisted that: "Nigerian leaders must take this opportunity now or answer to their children."
UN agencies have been giving polio drops to some 60m children in 10 West and Central African countries, as part of their goal to eradicate polio by the end of next year.
Northern Nigeria is at the centre of a resurgence of the disease and accounts for almost half of new cases worldwide.
A new case of polio has been reported in Ivory Coast, after more than three years of being polio-free.
The World Health Organisation is investigating whether it is the same strain of the disease as that found in Nigeria.
It has already spread to seven of Nigeria's neighbours.
Some studies have shown the vaccine to be safe but one report released last month found traces of the reproductive hormone oestrogen.
The World Health Organization, which denies the clerics' claims, called an urgent meeting last month to urge countries where polio is still endemic to eradicate the disease.
Health experts point out that children elsewhere are no longer immunised, so if the disease is not contained it could spread rapidly around the world.
Nigeria's health ministry has set up a team of all concerned parties to witness tests on the vaccine in laboratories in South Africa, India and Indonesia. The group has now returned to Nigeria.
A team member told the BBC they were working hard to release their findings as soon as possible.
The disease, which once affected millions of children, attacks the central nervous system, often causing paralysis, muscular atrophy and deformity.
Between 5% and 10% of those infected die when their breathing muscles become paralysed.
It is usually contracted through exposure to contaminated water.
More than 75% of worldwide polio cases are linked to the five hotspots shown on the map
Six countries previously declared polio-free were re-infected in 2003