The country's disease control and prevention centre has said that H1N1 flu is now widespread in 46 of the US's 50 states.

'Proactive measure'

Seasonal flu normally peaks between late November and early March, but H1N1 appears to be peaking early and with more waves of infection expected.

An administration official said: "It's important to note that this is a proactive measure - not a response to a new development.

"H1N1 is moving rapidly, as expected. By the time regions or healthcare systems recognise they are becoming overburdened, they need to implement disaster plans quickly."

More than 20,000 Americans have been hospitalised with H1N1.

Separately, the World Health Organisation (WHO) has reported more than 414,000 laboratory confirmed cases of H1N1 worldwide, with nearly 5,000 deaths.

It also notes that those figures are likely to be "significantly lower than the actual number of cases that have occurred".