The World Health Organization says countries must not lower their guard in the response to the swine flu outbreak.
Almost 900 cases had been confirmed across five continents, the WHO said, and authorities had to remain vigilant.
Viruses increased and decreased in activity, it said, and it was too early to tell whether the outbreak had peaked where it emerged in Mexico.
The warning came after health officials in Mexico said that cases of the virus appeared to be declining.
In Mexico, just over 100 people are thought to have died from the swine flu strain, although only 22 cases have been confirmed.
But on Sunday Health Minister Jose Angel Cordova said that the virus appeared to have peaked between 23-28 April.
“The evolution of the epidemic is now in its declining phase,” he told a news conference.
The WHO said authorities should remain on alert.
The current “round of activity” might have peaked, WHO official Gregory Hartl said, but that did not mean it was over.
“There is a high possibility that this virus will come back, especially in colder periods,” he said.
Health experts in the US, meanwhile, say swine flu could soon be present throughout their country, as cases have been confirmed in more than half of all states.
Outside Mexico, the effects of the virus do not appear to be severe.
In other developments:
Mexican President Felipe Calderon accused unspecified countries of “taking discriminatory measures because of ignorance”, amid a row with China over the quarantine of 70 Mexicans in three Chinese cities
Ã¢â‚¬Â¢ El Salvador says it has confirmed its first two cases of the virus, AFP reports, citing the country’s health minister
Ã¢â‚¬Â¢ Egypt says it will continue slaughtering pigs as a precaution against swine flu, following clashes on Sunday with farmers that left 12 people injured
Late on Sunday – before the apparent confirmation from El Salvador – the WHO said it had found 898 cases of the virus across 18 countries.
Person-to-person transmission has been confirmed in six countries.
In the US, the number of confirmed cases rose from 160 to 244. Officials said this was because the results of lab tests were now coming through, rather than because of a new surge in cases.
But an expert from America’s Centers for Disease Control and Prevention said the virus was fairly widespread.
“Virtually all of the United States probably has this virus circulating now,” Dr Anne Schuchat said. “That doesn’t mean that everybody’s infected, but within the communities, the virus has arrived.”
She said she expected cases to become more severe and to lead to deaths. She stressed that this in itself would not be unusual as every year 36,000 people die in the US after contracting seasonal flu.
WHO food safety scientist Peter Ben Embarek, meanwhile, said increased surveillance was necessary after the virus was found to have infected pigs in Canada.
But he said there was no recommendation to cull animals, and pork remained safe to eat.
“From a consumer point of view there is no risk from consuming cooked pork products,” he said.