MEXICO CITY: The head of the World Health Organisation warned on Monday that swine flu could return with a vengeance despite Mexico's President Felipe Calderon insisting his country has contained the epidemic.

WHO chief Margaret Chan said in a newspaper interview that a second wave of the virus "would be the biggest of all outbreaks the world has faced in the 21st century", puncturing optimism emanating from the epicentre of the outbreak.

The diplomatic damage from the epidemic also reverberated with China denying it had discriminated against Mexican nationals after dozens were placed under quarantine over the weekend despite showing no signs of the flu.

Twenty-five people have died from swine flu, according to the WHO which estimates there are nearly 900 cases in 20 countries. All but one of the deaths have been in Mexico.

Calderon said Mexico had managed "to contain the epidemic" and was now "in a position to overcome" the A(H1N1) virus.

His health minister, Jose Angel Cordova, told a news conference the epidemic peaked between April 23 and 28 and was "in its phase of decline".

But Chan said the end of the flu season in the northern hemisphere meant that while any initial outbreak could be milder, a second wave could be more lethal, reflecting a pattern first seen with the 1918 Spanish flu pandemic that killed up to 50 million people worldwide.

"We hope the virus fizzles out, because if it doesn't we are heading for a big outbreak," Chan told the Financial Times, adding that it could re-emerge in the months ahead "with a vengeance".

"I'm not predicting the pandemic will blow up, but if I miss it and we don't prepare, I fail. I'd rather over-prepare than not prepare."

The UN agency last week raised its alert level to five, indicating that a global pandemic is imminent.

More countries are confirming cases every day with Italy and Colombia becoming the latest to join the list, while France announced two new confirmed cases.

Japan on Monday tripled the number of quarantine officers at Tokyo's Narita airport to try to detect cases at the start of a key holiday week.

And in the United States, the only other country to have recorded a death from the virus, officials said that more than half the 50 states had now confirmed cases.

US Health and Human Services Secretary Kathleen Sebelius echoed Chan's warning that the real test would come when the winter influenza season hits in a few months.

"One of the things that we know is that even if this current situation seems to be lessening, if we are cautiously optimistic, we really don't know what's going to happen when real flu season hits (together) with H1N1 virus," she told CBS television.

In China, the centre of the 2003 SARS outbreak, authorities have been accused of discriminating against Mexican nationals in its bid to stop the disease in its tracks.

Although no cases of swine flu have been reported on the Chinese mainland, one Mexican who stayed in a hotel in Hong Kong has tested positive.

A Mexican embassy official said there were nearly 70 of his compatriots had been quarantined across China including in Beijing, Shanghai and the southern city of Guangzhou even though they had demonstrated no flu symptoms.

China's foreign ministry spokesman Ma Zhaoxu said in a statement: "The relevant measures are not directed at Mexican citizens and are not discriminatory."

Myrna Elisa Berlanga, 31, a Mexican tourist who arrived in China on Saturday, questioned why she was being quarantined since no one on her flight from the United States had the virus.

"Right now we are tired of being confined, not knowing what will happen with us," she told AFP by phone from the hotel near Beijing's airport where she and nine other Mexicans are being isolated.