WHO chief Dr Keiji Fukuda said it was not inevitable that the outbreak would develop into a global epidemic - or pandemic - but countries should "take the opportunity to prepare".
Several countries have issued warnings against travelling to Mexico, but the WHO and the EU's health chief, Androulla Vassiliou, said such measures would do little to combat the flu's spread.
WHO spokesman Gregory Hartl said experts were working on a vaccine, but said it could take five or six months to develop.
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.
UN alert raised
Sixty-four cases have been confirmed in the US so far.
US President Barack Obama has asked Congress for $1.5bn (£1bn) to help prepare for a possible outbreak, and California Governor Arnold Schwarzenegger has declared a state of emergency over the threat.
Health officials in New Zealand, Israel, Canada, the UK and Spain have all reported a small number of cases in their countries.
WHO PANDEMIC ALERT PHASES
Phase 1: No infections in humans are being caused by viruses circulating in animals.
Phase 2: Animal flu virus causes infection in humans, and is a potential
pandemic threat.
Phase 3: Flu causes sporadic cases in people, but no significant
human-to-human transmission.
Phase 4: Human-to-human transmission and community-level outbreaks.
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.
Post-peak: Pandemic activity appears to be decreasing though second wave
possible. Post-pandemic: activity returns to normal, seasonal flu
levels.
BACK{current} of {total}NEXT
The EU said patients were also under observation in Denmark, Sweden, Greece, the Czech Republic, Germany and Italy.
Tests are being carried out on people in Brazil, Guatemala, Peru, Australia and South Korea.
Some countries in Asia, Latin America and Europe are screening airport passengers for symptoms, while Germany's biggest tour operator has suspended trips to Mexico.
Cuba also suspended flights to and from Mexico for 48 hours from Tuesday.
The UN's Food and Agriculture Organisation (FAO) is sending a team to Mexico to investigate rumours that people had been falling ill last month near intensive pig farms.
In Mexico City, officials have banned restaurants and cafes from serving all food except takeaways to help prevent the flu spreading.
The WHO raised its pandemic alert status to level four on Monday - two levels from a full pandemic - after concluding there had been sustained transmission between humans.
Levels five and six are reserved for when there is widespread human infection.
WHO spokesman Mr Hartl said there were a number of cases in New York "which appear to be human-to-human transmission".
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
He said that if the New York cases were confirmed, the WHO could raise its alert to level five.
The UN body is encouraging countries to begin intensive surveillance of possible infection and send as much information as possible to the WHO.
In Mexico, swine flu has been confirmed in 20 of the 152 known deaths.
In almost all cases outside Mexico, people have been only mildly ill and have made a full recovery.
Meanwhile, Swiss police said a container with animal swine flu samples exploded as it was being shipped on a train from Zurich to Geneva on Monday, injuring a woman.
Authorities said dry ice keeping the samples cold caused the explosion, but the incident posed no threat to humans.
Bookmark with:
What are these?