Dr Richard Roberts, head of Public Health Wales's vaccine preventable disease programme, said the measles outbreak is accelerating and although the "vast majority" of parents have their children vaccinated there is a small minority who have not had the MMR vaccine.
"It is the largest outbreak of measles that we have had for more than 10 years in Wales", he said.
Fresh measles cases have been reported in Powys and Cardiff. A public health expert warned tens of thousands of children could be at risk.
Public Health Wales has information and advice on measles - a highly-infectious viral illness which can have life-threatening complications.