The United States has proposed that the last known stockpiles of the smallpox virus should be retained for at least another five years to allow for more research and prevent one of the world's deadliest diseases being used as a biological weapon.

U.S. Health and Human Services Secretary Kathleen Sebelius told reporters Tuesday that the World Health Organization has been asked to decide whether the stockpiles held in secure U.S. and Russian labs should remain in place for at least another five years, when experts could again review the situation.

At a news conference at the U.N.'s European headquarters where the World Health Assembly also was debating the matter, Sebelius said the U.S. is "committed to the eventual destruction" of the stockpiles but fears that smallpox could still re-emerge and be released unintentionally or deliberately used as a biological weapon. Scientists would need the virus to create a vaccine.

An aide to Sebelius, Bill Hall, later told AP that the United States would not act unilaterally, but would await the outcome of a vote on the U.S. proposal in the Geneva-based assembly.

For centuries smallpox killed about one-third of the people it infected, but it was eradicated from the environment three decades ago. The last known case was in Britain in 1978.

Many countries say the world would be safer if the remaining stockpiles of the virus — held at U.S. federal facilities in Atlanta, Georgia, and a Russian research center in Koltsovo — were now destroyed. At the meeting this week some of WHO's member countries again pushed to set a date for the virus' destruction, the fifth year they have made such an attempt.

WHO officials said there was no indication there was more support for resolving the dispute this year.

The U.N. health agency first agreed in 1996 that smallpox should be destroyed — though it lacks the power to enforce the decision. But that has been repeatedly delayed to give scientists time to develop safer smallpox vaccines and drugs. There are now two such vaccines and a third coming along, plus experimental drugs in the works to treat but not cure the disease.

A report commissioned by WHO last year concluded the stockpiles could still be useful for drug development.

Sebelius said the possibility of a future biological threat demands more tests with the virus. She says no one has ever verified whether all nations complied with WHO's efforts to transfer all the collections of smallpox virus to the U.S. and Russian labs.

"The world has no immunity to smallpox whatsoever," she said. "It could be released unintentionally or released as a bioweapon."

Microsoft co-founder Bill Gates did not directly address the smallpox issue, but he told the world's health ministers Tuesday that every nation can increase its vaccination rates to as much as 90 percent coverage against meningitis, polio and other diseases over the next decade, as a low-cost way to protect their populations.

(This version CORRECTS Corrects the title and surname of the top U.S. official.)