Los Angeles (AFP) - A state of emergency was declared on Friday in the western US state of Washington following a measles outbreak that has affected more than two dozen people, the majority of them children.

The disease was declared eliminated in the US in 2000 but has since made a comeback that is tied to imported cases and the rise of the anti-vaccine movement.

"Measles is a highly contagious infectious disease that can be fatal in small children," Washington Governor Jay Inslee said in a statement. "The existence of more than 26 confirmed cases in the state of Washington creates an extreme public health risk that may quickly spread to other counties."

The outbreak began near Portland, Oregon, at the start of the year and quickly spread to nearby Clark County and King County, both in Washington.

Health officials have warned that people infected with the disease had visited schools, churches, a dentist's office, a Costco store, an Ikea store and the Portland airport.

The majority of those infected are children, many of whom have not been immunized against the disease, officials said.

They added the outbreak could still be in its infancy as the incubation period of the virus averages 14 days. Those infected can spread measles to others four days before and four days after the rash appears.

The highly contagious disease can cause severe diarrhea, pneumonia and vision loss, and ultimately can be fatal.

The World Health Organization in November warned that measles cases worldwide had jumped more than 30 percent in 2017 compared to the previous year, in part because of children not being vaccinated.