Geneva - The World Health Organization Friday rejected criticism that it was too slow in reacting to the outbreak of the swine flu A(H1N1) virus.

Mexican officials as well as some media reports have said there was too long a gap between information reaching the organisation that a new influenza was apparently emerging and the WHO's reaction.

"Once we knew this illness was caused by a new influenza virus, we moved into operation in a matter of hours," said spokesman Thomas Abrams. "This has been a pretty rapid response."

The WHO added that it was conducting internal reviews of its actions.

Late Thursday, Keiji Fukuda, the WHO's health security chief, explained that some time delay has be expected when dealing with a new virus.

"Of course there is a time delay," he said. "Most diseases do not come out with someone walking around with 'new disease' written on their forehead."

He noted that the new virus emerged towards the end of the Northern Hemisphere's normal flu season and therefore it also took some time for local authorities to realize something was awry, given that it was not unusual to see some spikes in the number of people reporting illness.

Meanwhile, the organization, which has dropped the use of the term "swine influenza" following consumer boycotts of pigs and pork products said it was now able to confirm 331 cases of the H1N1 virus.

It reported 156 laboratory confirmed cases in Mexico, including 9 deaths and 109 cases in the United States including 1 death. In both countries there was strong evidence of sustained community spread of the disease, which prompted the WHO on Wednesday to raise its pandemic alert to phase 5.

Other countries with confirmed cases were Canada, Britain, Germany, Israel, Austria, New Zealand and Switzerland.

Spain had a reported 13 cases, included one of a person who did not travel to Mexico.

The WHO said there was a huge backlog, particularly from Mexico, of samples waiting to be tested. As this was a new virus, Fukuda said, few laboratories had the capabilities to confirm the strain.

Fukuda said that while individuals and governments should remain alert, there was no need to panic. He did however warn about reading too much into fluctuations.

"All outbreaks go up and down," he said, referring to reports from Mexico that the number of new cases appeared to be dwindling. "This is the nature of influenza. If didn't go up and down it would be very unusual."

The UN health agency said it did not yet have figures on the funding that would be required to deal with the new virus.

The World Bank said it would give Mexico 205 million dollars for its work on the virus.

It was unclear how the WHO would react if the virus caused a large outbreak that required the distribution of significant quantities of anti-viral drugs.

The organization was in contact, however, with pharmaceutical companies, and Roche, the maker of Tamiflu said it would coordinate with the international body to make sure enough drugs were available.

The WHO said it would brief reporters later Friday on issues pertaining to vaccines for both seasonal flu and the new virus.