"It's very obvious that we are very concerned. We've stood up emergency operation centers," Centers for Disease Control and Prevention (CDC) spokesman Dave Daigle told AFP.

One major source of concern was that the virus included strains from different types of flu.

Swine Flu, Mexico Lung Illness Heighten Pandemic Risk

"This is the first time that we've seen an avian strain, two swine strains and a human strain," said Daigle, adding that the virus had influenza strains from European and Asian swine, but not from North American swine.

In 11 of 12 reported human cases of swine influenza (H1N1) virus infection in the United States from December 2005 to February 2009, the CDC has documented direct or indirect contact with swine.

But the seven known cases of the previously undetected strain in the United States -- five from CaliforniaTexas -- did not have contact with pigs. The seven people infected have all recovered from the flu. and two from

"We have determined that this virus is contagious and is spreading from human to human," the CDC said on its website. "However, at this time, we have not determined how easily the virus spreads between people."

Local and state health officials were interviewing not just the people who were infected but the people with whom they had contact, Daigle noted.

Officials were looking for the source of the infection, Daigle said, adding that US health officials were due to receive samples from Mexico that would be tested at a lab at the centers based in Atlanta, Georgia.

The World Health Organization (WHO) has identified swine influenza as a potential source of a human flu pandemic. Pandemics usually occur every 20 years.

"Our experts and others are saying are not saying it's not a matter of whether but when. And we are past due," said Daigle.

Swine flu is caused by type A influenza and does not normally infect humans but cases have been reported among people, especially those exposed to pigs, the CDC said. Most outbreaks take place during the late fall and winter months.

Swine flu symptoms include fever, lethargy, lack of appetite and coughing. Some people who have contracted the virus have also reported runny nose, sore throat, nausea, vomiting and diarrhea, according to the CDC.

Human outbreaks of H1N1 swine influenza virus were recorded in the United States in 1976 and 1988, when two deaths were reported, and in 1986. In 1988, a pregnant woman died after contact with sick pigs, the WHO said.

In recent years, the global focus for a pandemic has shifted to the H5N1 bird flu virus, which has spread from poultry to humans, especially in Asia.