A fourth state in northern Nigeria has pulled out of a mass immunisation programme in 10 West African countries.
Kano officials are still not happy with the vaccine
The state of Niger has joined its neighbours Bauchi, Kano and Zamfara in refusing to cooperate until, they say, the vaccine has been confirmed as safe.
A spokesman for the United Nations Children's agency Unicef described the decision as "very sad".
The immunisation drive, billed as the final effort to wipe out polio, will cover some 60m children in three days.
Nigeria accounts for a half of all new polio cases, mostly in northern Nigeria.
Thousands of volunteers have begun going from door to door to administer the oral vaccine; most parents are reported to be cooperating.
An observer in the Nigerian city of Kaduna spoke of mothers prising open the jaws of wailing infants to give them the three drops of vaccine.
Kano suspended immunisations following reports by Muslim clerics that the vaccine was contaminated with an anti-fertility agent as part of a US plot to render Muslim women infertile.
"Unless we are convinced by our committee (of health experts) that the oral polio vaccines are safe, the exercise remains suspended in Kano state," said Kano government spokesman Sule Ya'u Sule.
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.
"We risk reversing our gains. We can't eradicate polio globally if everyone does not take action," said United Nations Children Fund Abuja spokesman Gerrit Beger.
The programme is the start of monthly national immunisation days during the low season for polio.
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.
It is hoped that vaccinating children now, when the virus is at its weakest, will be the best way of stopping transmission.
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.
Health officials in both Kano and Zamfara states are reportedly training staff in expectation that the campaign will soon resume.
However, the continued suspension of the programme has caused concern.
A United Nations Children's Fund spokesman said any delay would result in the spread of the virus crippling more innocent children, both in Nigeria and neighbouring countries.
Polio has already radiated out from Kano to infect people in at least six west and central African states.
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