Also, school officials should be planning for online or distance-learning programmes in the event that schools do close, officials said.

Homeland Security Secretary Janet Napolitano said only schools with 'high numbers of high-risk students' showing symptoms should consider closing, but she warned that shutting down a school, even temporarily, 'causes a very significant ripple effect' in the community.

Although strains of the virus have emerged that are resistant to Tamiflu, one of two antiviral drugs effective in treating it, scientists say both drugs generally appear to continue to be effective.

The US government shipped 11 million doses of the drugs to states to add to the 23 million they already had on hand and bought an additional 13 million doses to replenish its supplies.

Officials are hoping to navigate a fine line, urging precautions to minimise spread, serious illness and deaths while avoiding undue alarm and misinformation.

'The strategy and the effort on the part of the governments is to make sure we ...collaborate to minimise the impact,' said John O. Brennan, the US deputy national security adviser for counter-terrorism and homeland security.

The US, Canada and Mexico have also vowed new vigilance against an expected resurgence of swine flu in coming months and said measures such as general border closures would be unlikely to prevent the spread of the virus.

On the contrary, they could only aggravate the economic and social consequences of a pandemic, US President Barack Obama, Mexican President Felipe Calderon and Canadian Prime Minister Stephen Harper said in a joint communique after their Aug 9-10 summit in Mexico.

(Arun Kumar can be contacted at arun.kumar@ians.in)