NEW YORK — New York City declared a public health emergency Tuesday over the measles outbreak in a predominantly ultra-Orthodox neighborhood and ordered mandatory vaccinations for those who may have been exposed to the virus.

Mayor Bill de Blasio announced the unusual order amid what he said was a measles “crisis” in Brooklyn’s Williamsburg neighborhood, where more than 250 people — mostly members of the ultra-Orthodox (or Haredi) community — have contracted measles since October.

>> Read More: War breaks out in New York's ultra-Orthodox community over measles outbreak

"This is the epicenter of a measles outbreak that is very, very troubling and must be dealt with immediately," de Blasio said.

The disease is easily spread and can be fatal, but there have been no confirmed deaths so far, officials said.

The order applies to anyone living, working or going to school in four ZIP codes in the neighborhood.

The declaration requires all unvaccinated people who may have been exposed to the virus to get the measles-mumps-rubella (MMR) vaccine, including children over 6 months old. People who ignore the order could be fined $1,000.

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.

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.

New York City’s health commissioner is empowered by law to issue such orders in cases where they might be necessary to protect against a serious public health threat.

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

“I want all New Yorkers to hear what I’m saying not just as your mayor but as a father: When my children were born, when they were young, all I thought about was how to keep them safe,” de Blasio said at the press conference in Williamsburg announcing the move. “We have a situation now where children are in danger. We have to take this seriously. Every one of us should feel responsibility for the situation.

“There’s no question that vaccines are safe, effective and life-saving,” he added.

The local outbreak began last fall when a visitor contracted measles during a trip to Israel, spreading the virus among unvaccinated people in Brooklyn’s Haredi community upon their return.

Last November, Haaretz reported that the outbreak was causing tensions among Brooklyn’s Haredi community.

Debra Nussbaum Cohen wrote that anti-vaxxers were being ostracized by fellow Haredim, quoting one Hasidic Jew, Alexander Rapaport, as saying that “People frown upon neighbors who aren’t vaccinating. You hear there’s someone in that building that doesn’t vaccinate, and now the whole building is having tsuris with them,” he said, using the Yiddish word for trouble.

Ultra-Orthodox rabbis outside of the city also issued edicts that parents should get their children vaccinated.

New York City Health Commissioner Dr. Oxiris Barbot assured the community on Tuesday that the MMR vaccine “is safe and effective. 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.

“We stand with the majority of people in this community who have worked hard to protect their children and those at risk,” she said. “We’ve seen a large increase in the number of people vaccinated in these neighborhoods. But as Passover approaches, we need to do all we can to ensure more people get the vaccine.”

Of the 285 confirmed cases to date, 246 were children and another 39 were adults. Some 21 cases have been hospitalized, with five admitted to the intensive care unit. Although measles can be deadly, there have been no fatalities in the latest outbreak to date.

The New York cases are part of a larger resurgence of measles in the United States. According to the Center for Disease Control and Prevention, a total of 387 individual cases have been confirmed in 15 states from January 1 to March 28 — the second-greatest number of cases reported in the United States since measles was declared eliminated there in 2000.

The highest reported number of cases since 2000 was 667 in 2014.

“When people choose not to get their children vaccinated, they are putting their children and others — such as pregnant women, people on chemotherapy, and the elderly — at risk of contracting measles,” said New York’s deputy mayor for health and human services, Dr. Herminia Palacio.

“While the MMR vaccine is the safest and most effective method of preventing measles, it is only 97 percent effective so population-wide immunity is a key component to protecting our most at-risk New Yorkers from measles,” the Mayor’s Office wrote in a statement. “Pregnant women — even if they have received the MMR vaccine — are still at risk of complications, including birth defects or loss of pregnancy.”

Another Jewish religious community, north of the city but with close ties to Brooklyn, has also seen a measles 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.

Reuters contributed to this report.