The number of measles cases in the United States has reached a 25-year peak, propelled by the spread of misinformation about the vaccine that can prevent the disease, federal health officials say.

The US Centres for Disease Control and Prevention reported 704 cases as of 26 April, a 1.3 per cent increase since the most recent tally of 695 reported on Wednesday.

The vast majority of cases have occurred in children who have not received the measles-mumps-rubella vaccine, which confers immunity to the disease, officials said on Monday.

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"The suffering we are seeing today is completely avoidable," US Health and Human Services Secretary Alex Azar said.

"We know vaccines are safe because they're among some of the most studied medical products we have."

A vocal fringe of US parents refuses to vaccinate their children believing, contrary to scientific evidence, that ingredients in them can cause autism or other disorders.

Some 22 states have recorded cases of the extremely contagious and sometimes deadly disease.

None of the victims of the recent outbreak have died, but three per cent have contracted pneumonia and nine per cent have been hospitalised due to complications from the disease, CDC director Robert Redfield said on Monday.

US President Donald Trump urged Americans last week to get vaccinated to prevent the spread of measles, changing course from remarks he made in 2014 when he expressed doubt about giving children government-recommended doses of vaccines.

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"The vaccinations are so important. This is really going around now," Trump said on Friday.

The current outbreak has been concentrated in New York City, where officials said more than 390 cases have been recorded since October, mostly among children in Orthodox Jewish communities in Brooklyn.

Most of the recently recorded cases have been in New York and Los Angeles, officials said on Monday.