Washington - President Barack Obama said he was closely watching an outbreak of swine flu in the United States, where 20 cases have so far been identified, but urged people to remain calm.

"This is obviously a cause for concern and requires a heightened state of alert, but it's not a cause for alarm," Obama said in a speech at the National Academy of Sciences in Washington.

Obama said he was getting "regular updates" from US authorities about the disease, which is believed to have killed more than 100 people in Mexico and prompted health alerts across the globe.

US officials on Sunday said there were 20 confirmed cases of swine flu in five states in the United States, but no deaths have been reported. (dpa)