US President Barack Obama and other top officials urged Americans to ramp up preparedness against swine flu, warning that the virus could return with a vengeance in the fall.

"I think it's clear that although we were fortunate not to see a more serious situation in the spring when we first got news of this outbreak, the potential for a significant outbreak in the fall is looming," Obama.

"We want to make sure that we are not promoting panic, but we are promoting vigilance and preparation," he added in remarks piped into a flu preparedness summit here from a summit of the Group of Eight industrialized powers in Italy.

Health Secretary Kathleen Sebelius warned at the meeting in the Washington suburb of Bethesda, Maryland that the A(H1N1) flu virus "is not gone, it's continuing to spread, it's in more than 100 countries around the world and in every state in this country."

And, she warned, US scientists and health officials who have been monitoring the virus as it works its way through the southern hemisphere say the virus "could worsen in the fall or earlier, when schools start to open."

The White House has drawn up a battle plan for taking on the virus when influenza season returns to the northern hemisphere in several weeks' time.

A key facet of the four-tiered plan is vaccination.

"We know that a safe and effective vaccine is the best means of both preventing the disease in individuals and stopping the community spread of the virus," said Sebelius, saying she expected to be ready to launch a mass vaccination campaign in October.

Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said clinical trials on a first candidate vaccine were expected to begin next month.

The tests would take "a couple of months" to determine whether the vaccine is safe, induces a protective response and what the right dosage and number of doses would be, he said, adding he also expected results in early October.

Pharmaceutical companies are not waiting for the results of the clinical trials to come back but are "already revving up and making a lot of doses to be able to administer," said Fauci.

A handful of pharmaceutical companies around the world are working to develop a vaccine against A(H1N1) influenza, which the World Health Organization (WHO) says has infected nearly 100,000 people in 137 countries and territories, and caused over 440 deaths worldwide.

Fauci said populations who have been shown to be at the greatest risk from the new strain of swine flu would be the first in line to be immunized.

"It's likely that a high priority will be given to children because the epidemiology of this virus is preferentially affecting younger people," said Fauci.

Pregnant women, people with suppressed immune systems and healthcare workers would also be in the front row for vaccinations, he said.

While all the officials urged Americans to take steps to limit the spread of flu in their homes and at work -- such as frequent handwashing, staying home when ill or covering a cough with a shirtsleeve -- Fauci ruled out more draconian measures, such as those implemented in other countries.

China, South Korea, France, Egypt, Singapore, Japan and the United Arab Emirates are among countries that have quarantined people -- many of them Americans or travelers who transited through the United States -- or checked their temperatures, over fears of swine flu.

"We don't put a lot of restrictions on people and quarantine them and keep people in hotels, because we don't think those kinds of public health measures are efficient when you have such widespread infection," said Fauci.

"We have so much infection in the United States that I think that quarantine measures don't work. I think we have just the right balance of measures here."

The United States has the highest death toll from swine flu of any country in the world, with 170 dead and nearly 34,000 confirmed cases. In Mexico, where the outbreak was first reported in April, 121 people have died from the illness.

US officials believe a million people in the United States may already have had swine flu, but did not seek medical care because the infection was mild.