US President Barack Obama has ordered his administration to take active and aggressive measures to deal with swine flu, which has infected at least 20 people in the United States.

As one of the measures, the administration declared a public health emergency on Sunday.

There are 20 cases have been confirmed of the epidemic in the United States, said Richard Besser, acting director of Center for Disease Control and Prevention, at a press briefing held in the White House on Sunday.

According to Besser, there were eight confirmed cases in New York City, seven in California, two in Texas, two in Kansas and one in Ohio.

"As we look for cases of swine flu, we are seeing more cases of swine flu. We expect to see more cases of swine flu," said Besser.