A US health official on Saturday did not rule out the possibility that the A(H1N1) influenza virus outbreak may have originated in California.
"As we do our investigations here in the US, we may find that there were cases earlier," US Centers for Disease Control and Protection (CDC) spokesman Scott Bryan told AFP.
Several cases of infection were reported in California before the outbreak in Mexico, the epicenter of the swine flu outbreak, where the government has confirmed 427 cases of the disease as well as 16 deaths.
The number of confirmed US swine flu cases stands at 160 spread across 21 states, including one death.
The first case discovered in California was a 10-year-old boy in San Diego County, which borders Mexico, who became ill on March 30 -- before the outbreak in Mexico, The Wall Street Journal reported.
A throat swab taken from the boy was sent for analysis to the CDC, which received samples on April 13 and determined the cause to be the A(H1N1) virus.
In a second case, a nine-year-old girl from a nearby California county was treated for a cough and a high fever on March 28. On April 17, tests confirmed that the girl did indeed have the new virus strain, the Journal reported.
The children, who had not traveled to Mexico or come into contact with swine, have since recovered.
Bryan did not deny the specifics of the California cases, but noted that from December 2005 to January 2009, there were 12 atypical strains tested at the CDC that had the virus.
The potentially deadly virus is a hybrid drawn from strains found in pigs, birds and humans.
About a third of the US cases have had contact with Mexico, CDC interim deputy director for science and public health programs Anne Schuchat told reporters Saturday.
Mexico meanwhile said its flu outbreak appeared to be stabilizing.
"I believe we have enough elements to say that we are in a stabilization phase," said Mexican Health Minister Jose Angel Cordova.