The flu outbreak has left streets in Mexico City almost deserted
Swine flu in Mexico, the epicentre of the world outbreak, has peaked, the Mexican health minister has said.
Jose Angel Cordova said the virus, blamed for at least 19 deaths in Mexico, appeared to have peaked between 23-28 April.
"The evolution of the epidemic is now in its declining phase," Mr Cordova told a news conference.
World Health Organization officials said authorities should remain vigilant as the virus could return.
Asked about Mr Cordova's comments, WHO official Gregory Hartl said the current "round of activity" could have peaked.
But he added: "We cannot lower our guard. There is a high possibility that this virus will come back, especially in colder periods."
The WHO says it has found swine flu in 787 people in 17 countries, while collated national figures give a tally of more than 800.
Person-to-person transmission has been confirmed in six countries.
In Mexico, just over 100 people are thought to have died from the virus, though only 19 cases have been confirmed.
Mexico: 101 suspected deaths - 19 confirmed
US: One death, 226 confirmed cases
New Zealand: 4 confirmed cases
Canada: 85 confirmed cases
Spain: 20 confirmed cases
UK: 16 confirmed cases
Germany: 6 confirmed cases
Israel: 3 confirmed cases
France: 2 confirmed cases
Netherlands, Switzerland, Austria, Denmark, Hong Kong, South Korea, Italy, Irish Republic, Costa Rica, Colombia: 1 confirmed case
Countries with confirmed cases of secondary transmission:
Mexico, US, Canada, Spain, Germany, UK
The first cases surfaced at the end of the northern hemisphere's winter flu season, and experts fear that infections could accelerate in the autumn.
WHO food safety scientist Peter Ben Embarek 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.
The herd of pigs in the western Canadian province of Alberta was apparently infected by a farm worker who recently returned from Mexico.
Mr Ben Embarek said the infection of the pigs was not a surprise and that both the animals and the worker were recovering.
In Egypt, the government moved to cull all the country's pigs following the swine flu outbreak.
There have been no cases of swine flu in Egypt, and the decision triggered protests from Coptic Christian pig farmers.
Meanwhile the US Centers for Disease Control and Prevention (CDC) said the number of confirmed cases in the United States had risen from 160 to 226 and that it had been found in 30 states.
Most people have not fallen seriously ill.
However, Dr Anne Schuchat from the CDC told reporters the virus was now probably circulating across the US.
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.
The BBC's Jon Donnison in Washington says most Americans are still not sure how concerned they should be about swine flu.