NEW YORK (AP) - New York City declared a public health emergency Tuesday over a measles outbreak centered in an ultra-Orthodox Jewish community and ordered mandatory vaccinations in the neighborhood.

Mayor Bill de Blasio announced the unusual order amid what he said was a measles crisis in Brooklyn's Williamsburg section, where more than 250 people have gotten measles since September. Officials blamed the outbreak on "anti-vaxxers" spreading false information.

The order applies to anyone living, working or going to school in four ZIP codes in the neighborhood and requires all unvaccinated people at risk of exposure to the virus to get the vaccine, including children over 6 months old.

The city can't physically force someone to get a vaccination, but officials said people who ignore the order could be fined $1,000. The city said it would help everyone covered by the order get the vaccine if they can't get it quickly through their regular medical provider.

"If people will simply cooperate quickly, nobody will have to pay a fine," de Blasio said.

Officials say 285 measles cases have been confirmed in New York City since the beginning of the outbreak, the largest in the city since 1991. New York City accounted for about two-thirds of all U.S. measles cases reported last week.

Ordering people to be vaccinated without their consent is "an extreme measure which is not provided for in the law and raises civil liberties concerns about forced medical treatment," Donna Lieberman, executive director of the New York Civil Liberties Union, said in a written statement.

The majority of religious leaders in Brooklyn's large Orthodox communities support vaccination efforts, said the city's health commissioner, Dr. Oxiris Barbot, but rates have remained low in some areas because of resistance from some groups that believe the inoculations are dangerous.

"This outbreak is being fueled by a small group of anti-vaxxers in these neighborhoods. They have been spreading dangerous misinformation based on fake science," Barbot said. "We stand with the majority of people in this community who have worked hard to protect their children and those at risk. We've seen a large increase in the number of people vaccinated in these neighborhoods."

Officials also noted that Passover is approaching, meaning increased travel among people who could carry measles to or from New York. Israel, for instance, also has a current measles outbreak.

The commissioner is empowered by law to issue such orders in cases when they might be necessary to protect against a serious public health threat.

News of the order got a mixed reaction in Williamsburg, with some residents - even those who support vaccination - saying they felt uncomfortable with the city pushing vaccines on people who don't want them. Others remain convinced, against expert assurances, that vaccines are unsafe.

"It's true that a lot of people have measles and measles are not a very good thing, but I think the vaccine also not a very good thing," said Aron Braver, a neighborhood resident. "And it's everybody's option to do what he wants. What he decides."

Earlier this week, the city ordered religious schools and day care programs serving that community to exclude unvaccinated students or risk being closed down.

Rabbi David Oberlander, director of the Yeshiva Kehilath Yakov School, where there were 20 measles cases, said "maybe 3%" of the students were unvaccinated.

"However, we worked very hard, as the health department told us, and those children were excluded," he said.

Another Jewish religious community, north of the city but with close ties to Brooklyn, has also seen a surge, with at least 166 cases since October. Last week, a state judge blocked an attempt by Rockland County officials to halt the spread of measles by banning unvaccinated children from public places.

The Centers for Disease Control and Prevention recommends that all children get two doses of measles vaccine. It says the vaccine is 97% effective.