Two children in California have been diagnosed and recovered from a new type of swine flu, U.S. health officials said Tuesday.

The cases are raising questions given that the 10-year-old boy in San Diego County and the nine-year-old girl in neighbouring Imperial County apparently had not been exposed to pigs or each other before they were infected with the H1N1 subtype of influenza A.

The children were diagnosed last week.

An investigation is underway and the World Health Organization and officials in Canada and Mexico have been informed, officials with the U.S. Centers for Disease Control said.

Both children became sick in late March and experienced fever and coughing. The boy also vomited.

Some of the boy's relatives also had flu-like symptoms, but they were not tested for the virus.

It's not clear whether a swine version of H1N1 is circulating widely or if it could trigger a pandemic in humans, the CDC said.

Human swine influenza on the rise in U.S.

While the U.S. agency has received reports of about one case of human swine influenza infection every year or two, more than 12 such cases have been reported since late 2005 — mostly among people exposed to pigs.

The girl went to an agricultural fair four weeks before she got sick, but her family said she did not touch a pig.

It's thought that better and broader diagnostic testing as well as genetic mutations could be fuelling the uptick in cases.

Early tests suggest this flu strain is resistant to amantadine and rimantadine, two common antiviral medications. A more common strain infecting people this flu season in Canada and the U.S. also does not respond to those drugs.

The current seasonal flu vaccine may not protect against this swine strain, health officials said.

There's no need for the public to take any unusual measures, the agency said in its alert to doctors and public health officials.

A senior official with WHO said they are watching the situation closely and there is no need to change the pandemic alert level.

With files from The Associated Press, The Canadian Press