President Donald Trump on Friday urged Americans to get vaccinated against measles as an outbreak spread across the country, reaching the highest number of cases since 2000.

"Vaccinations are so important," Trump told reporters at the White House. "They have to get their shots."

The start of the measles outbreak has been traced to Orthodox Jews who contracted the highly infectious disease while traveling abroad. Some of them are from communities that oppose getting vaccinations.

The resistance of some Jews to vaccinations on what they say are religious grounds is part of a wider "anti-vaxxer" movement in which people refuse to believe in the benefits of vaccinations or claim that the treatment is dangerous.

Officials say they have recorded 695 measles cases this year, the highest number since the disease was declared eliminated almost two decades ago.

Earlier this month New York's mayor declared a public health emergency in heavily Orthodox Jewish parts of Brooklyn, ordering all residents to be vaccinated to fight measles.

In the past, Trump has given some support to the anti-vaccination movement, claiming, for example, that heavy doses given to young infants may be linked to autism.