The number of American states reporting widespread flu activity have jumped from 23 to 36 last week, the US Centres for Disease Control and Prevention (CDC) has said.

"Seasonal influenza activity increased sharply in the US," the CDC said in a report covering the week ending December 23. "H3N2 viruses continue to predominate." "Widespread" means that more than 50 per cent of geographic regions within the state are reporting flu activity.