"The virus could cause a global (flu) outbreak. It was an unwise decision to send it out," said Stohr, who heads the United Nations health agency's influenza program.

But the laboratories, which are sent viruses so they can test their own capabilities in detecting flu strains, are experienced in handling such material. Most already had been alerted to the danger, so there was little chance of anyone catching it, Stohr said.

"It is a risk, but it is considered low. It should not lead to a big scare," Stohr said.

The virus, known as H2N2, was deadliest in 1957 and 1958, and people born after 1968 would have little immunity to it.

It was not certain whether all US recipients had been located yet. "There is more detective work to be done there," Stohr said. He added that the first batches were delivered as far back as October and no infections had been reported.

Outside North America, the microbes went to some 61 laboratories, and all of them had been contacted, Stohr said.

GLOBAL SCARE

Headlines about the mix-up raised concern in Washington and around the world.

"It's a high priority that the Centers for Disease Control (and Prevention) and the Department of Health and Human Services are working to address," said White House spokesman Scott McClellan.

"They have assessed that the risk to the public from these samples is low. Nevertheless, we do not want to take any chances," he added.

It was not immediately clear why the samples of H2N2 were distributed to labs. The US CDC was expected to make a statement later.

Cincinnati, Ohio-based Meridian Bioscience, which sent out the H2N2 virus samples, denied it violated any rules and said it was working on contract to the College of American Pathologists.

"Such samples are used by professional laboratories accustomed to handling viral agents. The company has a long history of supplying samples to the (college) and believes it has been and is in compliance with all applicable regulations," it said in a statement.

The College of American Pathologists issued instructions for all samples to be destroyed and planned to report to the WHO and US health authorities by Friday, Stohr said.

The latest alert comes as the WHO is already sounding the alarm over influenza because it fears that a continuing outbreak of the deadly bird flu in Asia, if not contained, could eventually trigger a human pandemic.

In an average year influenza kills up to 500,000 people globally and 36,000 in the United States alone.