Mexican President Felipe Calderon issued an emergency decree on Saturday giving the government special powers to run tests on sick people and order them isolated to fight the deadly flu crisis.
Mexico City has already shut schools and museums and canceled sporting and cultural events as an outbreak of a new type of swine flu killed up to 68 people in the country and spread north to infect some people in the United States.
Saturday's decree, published in Mexico's official journal, gives the government power to isolate sick people, enter homes or workplaces and regulate air, sea and land transportation to try to stop further infection.
The flu has rattled residents of Mexico's overcrowded capital of some 20 million people.
Calderon tried to calm Mexicans earlier on Saturday, saying the flue was curable. He said health authorities easily had enough antiviral medicine for the 1,000 or so people suspected to be infected with the swine flu and that his government was monitoring the situation "minute by minute."
Tests on Saturday showed eight New York City schoolchildren had a type A influenza virus likely to be the same type as the Mexican flu, adding to nine people in California and Texas who tested positive for it, although they later recovered. Two swine flu cases were also confirmed in Kansas.
The World Health Organization declared the outbreaks a "public health event of international concern" and urged all countries to boost their surveillance for any unusual outbreaks of influenza-like illness and severe pneumonia.
The agency stopped short of raising the threat level to a pandemic -- a global epidemic of a serious disease.
Mexico City residents mainly hunkered down at home on Saturday, as children's parties were canceled and bars were closed and many of those on the street wore surgical masks.