U.S. authorities are investigating two cases of swine flu in unrelated children in California.

Discovery of the two cases, in children who apparently had no contact with pigs or with each other, is raising questions about whether swine flu viruses are circulating there.

Officials of the U.S. Centers for Disease Control say they have informed the World Health Organization of the cases and the investigation that is underway.

The 10-year-old boy and nine-year-old girl were infected with influenza A swine viruses of the H1N1 subtype.

Officials say it’s not yet clear whether a swine H1N1 virus could trigger a human pandemic, because there is a lot of immunity in humans to human H1N1 viruses.

A senior official with the WHO says the organization is following the situation closely but there is no need to change the pandemic alert level at this point.