The number of confirmed human swine flu cases has risen to 28 in New York City, but those infected with the virus are from one school, Mayor Michael Bloomberg said on Monday.

"This in no way indicates a change in the size of the population affected," Bloomberg told a press conference. "It just means that we have more data back confirming what already suspected ... yesterday."

He said every one of the 45 confirmed or probable cases was associated with St. Francis Preparatory School in Queens, adding that "nearly all of the St. Francis students with confirmed swine flu are improving."

Eight students were confirmed to have been infected with the virus on Sunday.

"Four to five days after seeing the first signs of swine flu at Queens, we are still dealing with a single cluster of swine flu cases, all associated with this one school," he said.

Bloomberg said tests on six students from a daycare center in the city's Bronx district have all turned out to be negative.

"No other cluster is evident in New York City," he said.

Around the city, Bloomberg said, no intensive care unit had reported a single person with severe illness associated with even possible or suspected swine flu.

"That indicates that so far, we are not seeing a situation comparable to that being reported in Mexico," he said.

The 20 new confirmed cases in New York brought the total of the United States to 40. The other confirmed cases were seven in California, two in Texas, two in Kansas and one in Ohio.

In Washington, CDC acting director Richard Besser said that of the 40 cases, authorities were aware of only one that was hospitalized.

The patients, ranging from 7 to 54 years, have a median age of 16 years, he said.

Besser warned again that he expects broader range in terms the severity of the infection and cases in other parts of the United States.

"Thankfully so far, we've not seen severe diseases in this country as has been reported in Mexico," he said.