Public health officials in California on Sunday warned of the potential for a swine flu pandemic as the death toll of the disease increased in Mexico and cases cropped up in the United States.

A total of seven cases have been reported so far in California, with four in San Diego County and three in Imperial County, officials said.

California Governor Arnold Schwarzenegger has opened an emergency operations center, where public health officials are monitoring the progress of the virus and coordinating their response with CDC officials.

The latest victim in California, a 35-year-old woman diagnosed April 4, has since fully recovered, reports said.

Meanwhile, two other cases have been reported in Kansas and three in Texas. Scientists believe what makes the outbreak different is that the disease spreads this time through human-to-human contact, rather than human-to-pig contact.

No swine-flu-related deaths have been reported in the United States, but the outbreak has been much worse in Mexico, where more than 1,300 people are believed to have been infected, and 81 related deaths reported.

Mexico City, capital of the country, has been reportedly virtually shut down in an attempt to stop the flu's spread, and many of the city's 20 million people have taken to wearing surgical masks.

Health officials said containing the outbreak is unlikely as an effective vaccine has not been developed so far for the swine flu virus.

The World Health Organization declared the outbreak a "public health emergency of international concern," but said it was too early to predict whether a pandemic would occur.