"The situation is fluid and the situation continues to evolve."
Ban Ki-moon, the UN secretary general, said on Monday he was "concerned that this virus could cause a new influenza pandemic".
"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."
The United Nations' food agency on Tuesday said it was sending animal health experts to Mexico to check if the new strain of flu virus widely described as swine flu was really directly linked to pigs.
"So far evidence that the new strain of influenza A virus has entered the human population directly from pigs has not been established," the Rome-based Food and Agriculture Organisation (FAO) said.
Besides Mexico, swine flu cases have been reported in the United States, Canada, Europe, New Zealand and Israel.
Israeli radio on Tuesday confirmed that a man returning from Mexico had contracted the virus and was hospitalised.
New Zealand's government on Tuesday confirmed three cases.
The US has confirmed 44 cases, with 20 new cases from a New York school where students had returned from a school trip.
Two students at a school in California on Monday were confirmed to have swine flu, health officials said, bringing to 13 the number of known cases in the state.
Richard Fielding, a clinical psychologist in public health at the University of Hong Kong told Al Jazeera: "Concern is warranted at this stage but panic is not.
"The media have a role. One sees it [swine flu] described as a 'deadly influenza epidemic'. I think balanced reporting is important.
In the UK, authorities said two people in Scotland had tested positive for swine flu infection and were being treated in isolation in a hospital near Glasgow, although both were said to be "recovering well".
Australian health authorities said they were investigating 17 possible cases of swine flu on Tuesday.
Chile reported eight possible cases of swine flu on Monday, officials said.
"It is most likely that the tests ... will come back negative," Ricardo Quezada, director of a Santiago hospital where four of the suspect cases are being investigated, said.
Only Mexico has reported deaths from the virus.
Countries across the world are acting to halt the spread of the new strain, apparently a mix of viruses that affect humans, pigs and birds.
Airlines flying in and out of Mexico's international airport have started to check passengers for symptoms, and many are wearing masks on board.
Some airlines flying out of the US are also issuing face masks.
Mexico has closed all its schools until May 6.
The US government has declared a public health emergency and is advising against non-essential travel to Mexico, a call echoed by Androulla Vassiliou, the European Union's health chief.
"I would try to avoid non-essential travel to the areas which are reported to be in the centre of the cluster," Vassiliou said on Monday.
Vassiliou did not mention the US, and a senior US health official said he believed it was "premature" to urge people not to travel to the US.
Richard Besser, acting director of the US centre for disease control and prevention, said there had been only one hospitalisation for swine flu in the US that officials knew about.
Barack Obama, the US president, said that officials were monitoring cases.
"This is obviously a cause for concern and requires a heightened state of alert. But it is not a cause for alarm," he said.
People are also being tested in France, Italy, Sweden and Switzerland. Other suspected cases are being investigated in Brazil, and South Korea.
Deaths: At least 20 confirmed, 149 now suspected in Mexico
Sickened: 1,614 suspected or confirmed in Mexico, 44 confirmed in US, six confirmed in Canada; two in UK, suspected cases in Chile, New Zealand, South Korea, Australia, Spain, Brazil, France and Israel
Safety measures in Mexico: Schools closed until May 6, surgical masks given to train passengers, public events cancelled
Safety measures worldwide: Airports screening of travellers from Mexico for flu symptoms
Stocks of influenza medication increased in Europe, North America and the Middle East
Several countries including US and EU issue travel advisories for Mexico
Countries with partial or total bans on pork imports as a consequence of swine flu: China, Indonesia, Lebanon, Russia, Thailand, and Ukraine.
Russia, Hong Kong and Taiwan have said travellers returning from flu-affected areas who display symptoms will be quarantined.
Several countries have banned imports of pork and pork products from Mexico and the three US states reporting cases of swine flu, although scientists say the disease cannot be contracted by eating pork.
Despite the raised alert level, the WHO has recommended that countries do not close their borders or restrict travel.
Should swine flu escalate into a pandemic, the world is thought to be better prepared to cope with it than it has been with other viruses.
After the outbreak of Severe Acute Respiratory Syndrome (Sars) in 2003, which killed nearly 800 people, and amid regular reported cases of bird flu, countries have stepped up preparations against a pandemic, including stockpiling anti-viral drugs.
Worldwide, seasonal flu kills between 250,000 and 500,000 people in an average year. The new strain is worrying as it spreads rapidly between humans and there is no vaccine for it.
Most of the swine flu fatalities were aged between 20 and 50. A hallmark of past pandemics has been the high rate of fatalities among young adults.
The World Organisation for Animal Health says the virus is a mix of avian, swine and human viruses and has not been isolated in animals yet. It has recommended the name "North American influenza" instead of swine flu.