A new type of swine flu has infected at least two children in California and while both have recovered, U.S. health officials said on Tuesday they were looking for more cases.

They say it is possible the children were infected by other people and not by pigs, and said they have consulted with officials in Canada, Mexico and at the World Health Organization although there is no evidence that the new virus is circulating widely.

In a special alert, the CDC asked doctors in California's San Diego and Imperial counties, on the border with Mexico, to test anyone with flu-like symptoms and send the samples in for testing.

"Both of these kids came to our attention because they were seen in clinics which do routine surveillance for influenza infections," the CDC's Dr. Lyn Finelli told reporters in a telephone briefing.

Neither child, a 10-year-old boy and a 9-year-old girl, had especially severe symptoms, although the girl had had a fever of 104 degrees, Finelli said.

"We are not making the assumption that there are widespread infections out there and there is not any action that the public should take right now," Finelli said.

The influenza strain is an H1N1, the same family as one of the seasonal flu viruses now circulating. But it genetically resembles a virus found in pigs and not in people, officials at the U.S. Centers for Disease Control and Prevention said.

It has never been seen before, the CDC said.

"Neither child had known contact with pigs. The source of infection is unknown," they added. "The lack of known exposure to pigs in the two cases increases the possibility that human-to-human transmission of this new influenza virus has occurred."

CONTACT TRACING

In the past two or so years, Finelli said, 12 known cases of infection with swine flu have been reported but 11 of them followed known contact with pigs.

People were near the children were being questioned by CDC investigators and tested if they remember having been sick recently. The boy flew to Texas with his brother and is still there, the officials added.

Blood tests may show if people were infected in the past with the same virus.

The virus is usually fairly mild but it still kills between 250,000 and 500,000 people in an average year. And every few decades, a completely new strain pops up and it can cause a pandemic, a global epidemic that kills many more than usual.

Flu experts are afraid a pandemic could come at any time and are monitoring for new strains. One big fear is that the H5N1 avian flu virus now mostly affecting flocks in Asia, the Middle East, Europe and Africa might mutate into a form that could cause a pandemic.

It currently only rarely infects people but has killed 257 out of 420 infected in 15 countries since 2003, according to the World Health Organization.

CDC officials said that they do not believe these two cases represent the beginning of such a pandemic.