The U.S. State Department plans to issue a travel warning later Monday urging Americans to avoid all "nonessential" travel to Mexico because of an outbreak of swine flu, a U.S. official said.

Swine flu has killed 103 people in Mexico and has spread to the United States. Spain has reported one case of the virus, the first to be confirmed in Europe.

"There will be a travel warning urging Americans to avoid all nonessential travel to Mexico because of the swine flu," said a U.S. official, who spoke on condition he not be named as the warning has not yet been announced.