Health experts say the virus comes from the same strain that causes seasonal outbreaks in humans but also contains genetic material from versions of flu which usually affect pigs and birds.
A team from the UN Food and Agriculture Organisation is due to leave Rome for Mexico on Tuesday to investigate allegations that industrial pig farms in Mexico were the source of the outbreak.
Its chief veterinary officer, Joseph Domenech, told the BBC that rumours that people had been falling ill last month near some intensive pig farms meant the FAO had to act.
"[We had] no indication of human cases of direct contact with pigs but this can never be totally sure and the probability that this virus could come from pigs directly could not be anyway ruled out totally," he said.
'Decline in cases'
The number of flu cases under observation in Mexico has reached 1,614. Swine flu was confirmed in 20 of the 152 deaths.
Health Minister Jose Angel Cordova said all of those who had died were aged between 20 and 50. Infections among young healthy adults are a characteristic of past pandemics.
Mr Cordova said the first suspected case of swine flu had occurred in the southern state of Oaxaca but stressed that nobody knew "the point of origin or dissemination" of the virus.
He noted that the number of new cases reported by Mexico's largest government hospitals had declined during the past three days: from 141 on Saturday to 119 on Sunday and 110 on Monday.
Schools nationwide are to remain closed until 6 May as the country grapples with the outbreak.
In almost all swine flu cases outside Mexico, people have been only mildly ill and have made a full recovery.
In the US, a further 20 cases of swine flu were confirmed in New York. Cases have also been reported in Ohio, Kansas, Texas and California, bringing the total across the country to 50.
Swine flu is a respiratory disease thought to spread through coughing and sneezing
Symptoms mimic those of normal flu
Good hygiene like using a tissue and washing hands thoroughly can help reduce transmission
US and EU officials have urged caution for anyone considering travelling to Mexico.
Tests are being carried out on individuals or groups in Brazil, Guatemala, Peru, Australia and South Korea.
A number of countries in Asia, Latin America and Europe have begun screening airport passengers for symptoms, while Germany's biggest tour operator has suspended trips to Mexico.
Taiwan has announced it will quarantine any visitors showing symptoms of swine flu.
Several countries have banned imports of raw pork and pork products from Mexico and parts of the US, although experts say there is no evidence to link exposure to pork with infection.
Shares in airlines have fallen sharply on fears about the economic impact of the outbreak.
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