Geneva - The World Health Organization confirmed Monday 1025 officially reported cases of influenza A(H1N1) infection in 20 countries. The organization said it would be arranging a virtual conference of experts on Tuesday to look into the clinical aspects pertaining to the severity of the virus, Keiji Fukuda, the WHO's head of health security said.

"The severity of illness is one of the most important questions," Fukuda said.

Responding to journalists' questions about recent actions enacting quarantine measures against individuals, Fukuda said that such governmental moves could be acceptable in certain situations.

"Quarantine can be applied in specific situations and is quite reasonable to take," he said, avoiding comment on the ongoing occurrences, including the one in China.

Mexico, the UN health agency said, has reported 590 laboratory confirmed cases, including 25 deaths. The United States had 226 confirmed cases, including one death, while Canada reported 85 infections.

Elsewhere in Latin America, El Salvador reported two cases and Colombia and Costa Rica one each. In Europe, 11 nations reported confirmed cases, including 40 in Spain, 15 in Britain and eight in Germany. In Asia, Hong Kong and South Korea each confirmed one case, while Israel said it had three, the only cases in the Middle East.

The higher number of cases from Mexico, the WHO said, reflected ongoing testing of previously collected specimens, which has developed into a backlog of samples waiting diagnosis.

The UN has been trying to stress the need for governments and health professionals to stay alert and not become complacent, in spite of evidence showing the virus may not be as bad as originally feared.

Conversely, the WHO has been warning it is too early to say for certain what the nature of the virus will be over time and officials have not ruled out the disease may change for the worse.

With the Northern Hemisphere coming out of flu season and the south about to start its winter, Fukuda said "vigilance" was needed.

"Surveillance really needs to be strong everywhere," he said.

The WHO has kept its influenza pandemic alert at phase 5, which means there is evidence of sustained human to human transmission in at least two countries.

Outside of North America, however, the WHO did "not have evidence that the virus has taken hold and led to community level transmissions in any other countries right now," Fukuda said.

"There is some confusion about whether going to phase 6 says anything about the severity of the disease," he noted, adding that "these are separate issues and I hope everyone is clear about that."

"Let us remember that even if WHO does declare phase 6 - a pandemic - that would be a statement about the geographic spread of the virus, not its severity," UN Secretary General Ban Ki-moon said in New York.

"The bottom line is this: if and when the world faces a severe influenza pandemic, the UN system is ready," he said.

The new virus has shown a wide spectrum of impact, in most cases proving to be mild, but in Mexico having demonstrated its lethal side, though WHO officials say they do not yet know the reason for this.

The health agency has maintained its recommendation that eating well-cooked pork is safe and has no advisories on restricting travel, except for those already ill, who should seek medical consultation.

The WHO said it would give 2.4 million doses from its stockpiles of anti-viral drugs to 72 less developed countries, though it has not made public which ones would receive the donations.

Similarly, officials have not yet been able to put a price tag on dealing with the disease, though Mexico took a 205 million dollar loan from the World Bank to help its health-related efforts.