Many countries are not sufficiently prepared in the event of a global flu pandemic, say researchers.
Flu pandemics claimed millions of lives
US experts writing in the journal Science says that many countries may be vulnerable if a lethal strain emerges.
They say that vaccines and drugs should be stockpiled to guard against a severe outbreak - but this is not happening.
Richard Webby and Robert Webster, from St Jude's Children's Research Hospital in Memphis, said that a pandemic was inevitable - and may be imminent.
Pandemics in 1918 - the so-called Spanish flu - 1957 and 1968 killed many millions.
Recent years have seen particularly mild flu in many countries, and although this year's "Fujian" strain has posed more problems - particularly in Australia - experts say there is no sign so far this year of a major outbreak.
However, the researchers say that, should a deadly new strain emerge, the only way of protecting the public would be through mass vaccination - and the sheer quantities of vaccine required would be huge.
In the meantime, the progress of the illness would have to be checked by antiviral drugs.
However, this means that substantial quantities of these drugs would have to be stockpiled.
The researchers say that no country, including the US and UK, has been stockpiling anti-flu drugs for this purpose.
The emergence of Sars earlier this year, they say, shows the need to be able to respond swiftly to the threat of dangerous new viruses.
They write: "Pandemic influenza has already threatened twice in 2003.
"The events associated with these outbreaks show that we are in a much better position to respond rapidly to an influenza threat than we were in 1997.
"However, much remains to be accomplished. Overall, our state of preparedness is far from optimal."
Even if antivirals are available, they say, there would have to be the capacity to manufacture large quantities of vaccine as soon as one was developed. This is not currently present, they say.
"The world will be in deep trouble if the impending influenza pandemic strikes this week, this month or this year.
"It is now time to progress from talking about pandemic vaccines to taking action."
The World Health Organization is spearheading global efforts to prepare for a major flu outbreak.