London, Apr. 29 : The World Health Organisation (WHO) has warned that border controls may not be effective enough to stop the spread of swine flu, even as several countries tightened airport checks.

More than 80 Mexicans are believed to have died from pneumonia and respiratory illness linked to the virus.

Experience from previous episodes like the Severe Acute Respiratory Syndrome (SARS) outbreak, and from computer models, showed the strategy was ineffective at controlling outbreaks said Gregory Hartl, a WHO spokesman.

In a blunt assessment of the danger from swine flu, he said: "Border controls don''t work. Screening doesn''t work.

Hartl was speaking as many Asian countries affected by the SARS outbreak in 2003, which killed almost 800 people, set up thermal scanners at airports to screen for passengers who could be feverish.

On Monday WHO raised its pandemic alert level from phase three to phase four on the scale of one to six, indicating that it has begun to spread swiftly between humans.

The organisation''s pandemic preparedness guidance documents suggest countries should consider implementing exit screening - of departing passengers - at this stage.

"We''re prepared as if there were a pandemic," she told the NBC television channel. (ANI)