Washington ? At least four countries and more than 1,500 U.S. laboratories reported they had destroyed all their samples of a dangerous flu virus that had been shipped around the world, but thousands of others remained unaccounted for Wednesday as health authorities in 18 countries intensified efforts to prevent a deadly outbreak.

U.S. officials were investigating the cause of the mishap and racing to tighten restrictions on the handling of particularly dangerous flu strains while international health officials kept close watch for any sign of lab employees coming down with the flu.

The World Health Organization said Canada, South Korea, Hong Kong and Singapore had confirmed destruction of their samples, but it was unclear how quickly other nations would comply. Friday is the target date.

Although no infections had been reported and health authorities were hopeful the risk was low, they said they were taking no chances.

“We are doing everything we can to make sure that there’s no threat to human health,” said Julie Gerberding, director of the federal Centers for Disease Control and Prevention.

The same strain of virus, H2N2, killed 1 million to 4 million people, including 70,000 in the United States, in the 1957-58 Asian flu pandemic. The virus has not been seen outside labs since 1968.

Federal officials announced plans Wednesday to restrict the number of labs that can handle deadly flu viruses. “We will urgently be recommending a higher level of protection be used for any novel influenza virus,” Gerberding said. “We’re going to put this on a fast track.”