Health authorities in Yemen say 83 children have been diagnosed with polio, nearly double the figure reported just over a week ago.
A spokesman for the World Health Organization said the number of confirmed cases could exceed 100.
The outbreak comes despite a mass immunisation drive after the United Nations health body said Yemen was facing a major polio epidemic.
Two more immunisation programmes have been scheduled for May and June.
Sixteen previously polio-free countries have reported new cases since 2003, when a polio vaccine boycott in Nigeria was blamed for spreading the disease.
Polio mainly affects young children and can cause irreversible paralysis. Yemen was previously considered polio-free.
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.
Hardline Islamic clerics claimed the polio vaccination was part of a plot by the US to make people infertile or give them HIV-Aids.