The World Health Organisation says the swine flu virus has reached 160 countries and could infect two billion people within the next two years.
A senior WHO official says the virus is still in its early stages and will continue to spread for some time.
Assistant director general Keiji Fukuda said work on a vaccine was intensifying but safety could not be compromised by rushing the process.
The BBC reports the virus is thought to have killed almost 800 people.
Meanwhile, United States officials have given up trying to count how many people have swine flu, saying the best they can do is estimate the spread of the "unstoppable" virus.
The US Centers for Disease Control and Prevention has dropped its weekly H1N1 count.
Latest figures from the CDC show nearly 44,000 cases of swine flu have been confirmed in the United States, with just over 300 deaths.
