World health officials stepped up warnings of a possible pandemic of swine flu as governments released millions of antiviral drugs and the number of cases rose, spreading into Europe.
With the likely death toll in Mexico, the epicenter of the outbreak, now standing at 149, the World Health Organization (WHO) raised its flu pandemic alert level from three to four.
While the increase marked a "significant step towards pandemic influenza, it's also a phase which says we are not there yet," said Keiji Fukuda, acting WHO assistant-general for health, security and the environment.
Fukuda warned "that in this age of global travel where people move around in airplanes so quickly, there is no region to which this virus could not spread."
The number of confirmed cases in the United States more than doubled on Monday to 44 and Britain and Spain both said they had registered patients sick with swine flu, the first cases in Europe.
UN chief Ban Ki-moon warned the new multi-strain virus, which is believed to be a mix of a human flu virus and an avian flu that first came from swine, risked triggering a global pandemic.
"We are concerned that this virus could cause a new influenza pandemic. It could be mild in its effect or potentially be severe," Ban told reporters.
"We don't know yet which way it will go but we are concerned that in Mexico most of those who died were young and healthy adults."
Influenza caused three epidemics during the 20th century, the worst being the Spanish flu between 1918 and 1919 that killed at least 40 million people, according to the WHO.
Mexico, which was rocked by a 5.6 magnitude earthquake on Monday, said the number of confirmed and suspected deaths from the flu had now risen to 149, as other countries urged against non-essential travel to the tourism hotspot.
Schools across Mexico have been closed until May 6 in a bid to stop the spread of the flu, which has sickened more than 1,600 people.
"We're in the decisive moment of the crisis. The number (of deaths) will continue rising," Health Minister Jose Angel Cordova added.
Europe's first confirmed case was reported in Spain, where 26 suspected cases were being probed, while two people were found to have the disease in Britain and dozens more potential cases were being checked in seven European Union member states.
Fears the disease could further strain the already-embattled global economy gave stock markets the jitters, leading to falls in Europe and Asia, while trading on Wall Street was muted.
And oil prices fell sharply on fears the escalating outbreak could further dampen economic activity and impact energy demand.
Although the US government has declared a public health emergency with 44 cases in five states, President Barack Obama urged calm.
The swine flu outbreak "requires a heightened state of alert, but it is not a cause for alarm," he told a gathering of the National Academy of Sciences.
Richard Besser, acting director of the US Centers for Disease Control and Prevention (CDC), said the affected states of New York, Ohio, Kansas, Texas and California and others were receiving 11 million courses of antiviral drugs.
Besser stressed that of so far most people in the US had only been mildly ill.
Swiss pharmaceutical group Roche said it was ready to send out more stocks of the antiviral medication Tamiflu, some 220 million doses of which are in the hands of governments worldwide.
And British pharmaceutical giant GlaxoSmithKline also said it was "urgently" investigating how to boost production of its antiviral drug Relenza, as the race to develop a direct vaccine for the H1N1 strain gathered pace.
The company has already provided 100,000 packs of the drug to Mexican authorities, along with a further 170,000 doses of its seasonal flu vaccine.
The European Union called emergency talks of health ministers and advised against non-essential travel to areas where the deadly virus has surfaced.
The United States issued a travel alert warning against "non-essential travel" to Mexico.
In New Zealand the government said it was investigating a further possible 56 cases of swine flu, while one person in Peru and up to 12 in Canada are under observation with flu symptoms.
There have been six confirmed cases in Canada and in the Middle East, a 26-year-old Israeli was also hospitalized.