GENEVA (AFP) - - World Health Organisation chief Margaret Chan urged developing countries on Friday to be prepared for more severe cases of the "subtle, sneaky" swine flu virus.

"Countries especially in the developing world, where populations are most vulnerable, should prepare to see more than the present small number of severe cases," Chan told the 193 member states at the end of the WHO's annual assembly here.

The spread in impoverished nations was one of the signals that the WHO was keeping an eye on before it declares a pandemic, instead of simply relying on geographical spread under its influenza rulebook.

"What we will be looking for are something that will signify a substantial increase in the risk of harm to people," interim Assistant Director General Keiji Fukuda told journalists.

"This could be change in the clinical severity, it could be another kind of signal such as the impact on the southern hemisphere," he added.

Chan underlined that the new virus had been found and tackled in countries that could mobilise the best detection, marking the first time that the world could follow "the conditions conducive to a pandemic."

"The strength of a country's health system will make the biggest difference in sickness and survival during an influenza pandemic," Chan underlined.

The WHO director general also highlighted the risk that the A(H1N1) virus could intermingle with seasonal flu during the impending winter season in the southern hemisphere.

That meant they could "possibly exchange genetic material in unpredictable ways," she added, referring to the risk that the new virus could become more dangerous in the process.

Chan acknowledged a series of dilemmas in assessing the current threat and evaluating any possible declaration of a pandemic phase six.

"Phase five and six are virtually identical in terms of the actions they launch," she said in response to countries that have urged her to be cautious about moving to the highest level of the pandemic alert system.

"But even the best laid plans need to be fluid and flexible when a new virus emerges and starts changing the rules," she added.

The global healthy agency is leaving it up to individual countries to decide how best to tackle cases of A(H1N1) at home based on their local conditions and the pattern of the disease.

"We are in the early days yet and do not know enough to make sweeping recommendations," said Chan.

The new swine virus had notably proved so far to be less severe than the H5N1 avian flu that has sparked fears of a pandemic in recent years.

"This is a subtle, sneaky virus, it does not announce its presence or arrival in a new country with sudden explosion of patients seeking medical care or requiring hospitalisation," Chan said.

A WHO specialist pointed out on Thursday that most developing countries were simply not in a position to detect or track seasonal flu, let alone a potentially pandemic strain of swine flu.

But the UN health agency is helping poor countries to develop simplified means of detecting flu, such as by spotting clusters of unusual respiratory illness, a spokesman told AFP.

Most types of flu are more lethal for those who are weakened by other ailments. Poor countries carry the bulk of the global burden of disease, with impoverished and sparse medical cover.

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