The number of confirmed human swine flu cases in the United States has risen to 64, with at least five patients having been hospitalized, a health official said on Tuesday.

"I fully expect we will see deaths from this infection," as swine flu cases are investigated, said Richard Besser, acting director of the US Centers for Disease Control and Prevention in Atlanta (CDC).

According to Besser, the hospitalizations include three in California and two in Texas. Their ages range from seven to 54.

However, the CDC reported no new infection sites beyond five states already afflicted. Besser said there are 45 confirmed cases in New York city, 10 in California, six in Texas, two in Kansas and one in Ohio. However, at least four other cases have been reported by states.

"We are in a pre-pandemic period," the official said, adding that the CDC is stepping up laboratory cultivation of the swine flu virus for an eventual vaccine.