A polio epidemic in Yemen has leapt to more than 60 cases making it one of the world's worst outbreaks of the disease.
Some schools have been forced to close to prevent the spread of the disease
UN health officials say the infections will probably soon exceed the 100 mark, as more suspected cases are examined.
Polio mainly affects young children and can cause irreversible paralysis. Yemen was previously considered polio-free.
Sixteen previously polio-free countries have reported new cases since 2003, when a polio vaccine boycott in Nigeria was blamed for spreading the disease.
Experts suspect the strain could have been carried from Nigeria to other countries by migrant workers or pilgrims visiting Muslim holy sites in Saudi Arabia.
Hard-line Islamic clerics claimed the polio vaccination was part of a plot by the US to make people infertile or give them HIV/Aids.
"They are having a pretty big epidemic... But we should be able to stop the virus relatively quickly," said World Health Organization official Oliver Rosenbauer.
The UN health body is behind a national immunisation drive in Yemen which will take place at the end of May.
Six million vaccination packs are expected to arrive in Yemen next week, Mr Rosenbauer said.
WHO figures show 198 new cases of polio have been reported so far in 2005, most of them in Nigeria and Yemen.