The swine flu virus first detected in Mexico can no longer be contained and countries should focus on mitigating its effects, a top UN official said.
World Health Organization deputy chief Keiji Fukuda was speaking as the WHO raised its alert level to four, or two steps short of a full pandemic.
UN food inspectors are going to Mexico to examine reports that industrial pig farms were the source of the outbreak.
The number of probable deaths from the virus there has risen to 152.
The US, Canada, Spain, Britain and Israel have confirmed cases of the virus, but no deaths have been reported outside Mexico.
'Not inevitable'
Alert level four means the virus is showing a sustained ability to pass from human to human and is able to cause community-level outbreaks.
Mr Fukuda said this was a "significant step towards pandemic influenza" but a pandemic should not be considered inevitable.
Experts did not recommend closing borders or restricting travel, he stressed.
"With the virus being widespread... closing borders or restricting travel really has very little effects in stopping the movement of this virus," he said.
The first batches of a swine flu vaccine could be ready in four to six months' time but it will take several more months to produce large quantities of it, Mr Fukuda said.
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.
Travel advice
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.
US and EU officials have urged caution for anyone considering travelling to Mexico.
Swine flu officially arrived in Europe on Monday, when tests confirmed that a young man in Spain and two people in Scotland - all of whom had recently returned from Mexico - had the virus. They were said to be recovering well.
Tests are being carried out on individuals or groups in Israel, Brazil, Guatemala, Peru, Australia, New Zealand 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.
SWINE FLU
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
WHO PANDEMIC ALERT PHASES
Phase 1: No viruses circulating among animals causing infections in humans
Phase 2: Animal influenza virus causes infection in humans, and is considered potential pandemic threat
Phase 3: Influenza causes sporadic cases in people, but no significant human-to-human transmission
Phase 4: Verified human-to-human transmission able to cause community-level outbreaks. Significant increase in risk of a pandemic
Phase 5: Human-to-human transmission in at least two countries. Strong signal pandemic imminent
Phase 6: Virus spreads to another country in a different region. Global pandemic under way
Mexico: 152 suspected deaths from swine flu - 20 confirmed cases
US: 50 confirmed cases
Canada: 6 confirmed cases
Spain: 1 confirmed case
UK - 2 confirmed cases
Countries with suspected cases: Israel, Brazil, Guatemala, Peru, Australia, New Zealand and South Korea
