The World Health Organization is urging countries to be on the lookout for new flu viruses.

The WHO is stressing the importance of monitoring for new variants of flu and reporting any findings to the Geneva-based UN health agency.

The statement comes in the wake of a series of discoveries in the United States of human infections with a new swine-origin H3N2 virus.

There have been 10 infections with this virus in the U.S. since the new variant of swine H3N2 viruses was first seen in humans in July; some limited person-to-person spread is believed to have taken place.

The WHO says countries should remember they have an obligation under the International Health Regulations to notify the WHO of any human infections with a flu virus that is not one of the subtypes normally seen in people.

The International Health Regulations require countries to alert the global community to outbreaks of four diseases deemed to be an international threat: polio, smallpox, human influenza cases cause by novel flu viruses and severe acute respiratory syndrome or SARS.

The IHR, as the regulations are called, were devised after the 2003 SARS outbreak, which spread rapidly from China to a number of parts of the world -- including Canada -- before subsiding. An estimated 774 people died in the outbreak, 44 of them in Canada.

The WHO's statement also urges countries to send for further testing any influenza A viruses their national laboratories cannot identify. Influenza A viruses are the only types of flu viruses capable of causing flu pandemics.

Sporadic cases of human infection with flu viruses from animals occur from time to time, the WHO's statement says. "(They) are not considered unusual, and do not change WHO's current assessment of pandemic risk."

"However, because influenza viruses are unpredictable, they have the possibility to change and become more transmissible among humans as shown by the emergence of the influenza pandemic H1N1 virus in 2009," the agency suggests.

"For this reason, continued monitoring of the occurrence of human infections with these viruses and characterization of the viruses themselves are critically important to assess their pandemic potential."