Washington (AFP) - A US government laboratory mistakenly mixed a common flu strain with a dangerous and deadly type of bird flu and shipped it to another lab, the Centers for Disease Control said Friday.

The discovery came to light in recent days, and followed concerning reports about mishandled anthrax and forgotten smallpox vials at separate US government labs.

No one was exposed to the mixed flu strain, said CDC director Tom Frieden.

"These events should never have happened," said Frieden at a press briefing.

He said the incidents raise "serious and troubling questions," and added: "Frankly, I'm angry about it."

Frieden said he has issued a moratorium on the transfer of any biological samples, including infectious agents, within or outside the CDC until an investigation is complete.

He also called for appropriate disciplinary action for any staff members who knowingly violated protocol or failed to report a lab incident.

The CDC said it learned of the flu mix-up while it was finalizing a report about what happened with an anthrax incident on June 5, which it concluded was very unlikely to have exposed workers to dangers, though some 80 workers were initially considered vulnerable.

"Earlier this year a culture of non-pathogenic avian influenza was unintentionally cross-contaminated at the CDC influenza laboratory with the highly pathogenic H5N1 strain of influenza and shipped to a BSL-3 select-agent laboratory operated by the United States Department of Agriculture (USDA)," the CDC said in a statement.

"There were no exposures as a result of that incident."

The lab is closed until better safety measures can be put in place, and an investigation is under way.

"For me personally, this is the most distressing," said Frieden, who said he learned of it 48 hours ago, though the mix-up happened six weeks ago.

The H5N1 bird flu is highly contagious and has killed about 60 percent of humans who been sickened by it.

It first infected humans in 1997 during a poultry outbreak in Hong Kong, and became widespread in 2003 and 2004.

CHICAGO (Reuters) - The U.S. Centers for Disease Control and Prevention "may never know" how a fairly harmless form of bird flu was cross-contaminated with a dangerous bird flu strain before it was sent to a laboratory outside of the CDC, an agency spokesman said on Monday.

That's because most of the materials used in the experiment to culture the virus were discarded shortly after they were used by the scientists performing the work, which occurred in March, CDC spokesman Tom Skinner told Reuters.

The CDC disclosed the bird flu incident as part of an internal investigation into the agency's mishandling of live anthrax in June, potentially exposing dozens of its own lab workers to the pathogen.

While no humans fell ill as a result of the bird flu breach, CDC Director Dr Thomas Frieden has called it “the most distressing" in a series of safety breaches at the agency because of the public risk posed by the virus.

Researchers at a high-security CDC influenza lab learned of their mistake in May. The contaminated bird flu samples had been sent to poultry researchers at the U.S. Department of Agriculture, who noticed their chickens all died.

It took another six weeks before the incident was reported to top brass at the CDC in early July, triggering an outside inspection of CDC labs that concluded on Friday.

Federal investigators are trying to piece together how it was that the laboratory never reported the incident up the chain of command.

Skinner said a key regulatory violation occurred when the CDC failed to properly document what it sent to the high-security biocontainment lab at the USDA.

"We thought we were sending H9N2," a far less dangerous form of bird flu, Skinner said. "We didn't know it was cross-contaminated."

Skinner said cross-contamination often can occur if improperly disinfected instruments come in contact with a growth medium, the material used to grow up the organisms, or if infected growth medium is inadvertently used.

"The mediums and all of the materials that were used to grow up this particular virus - all of that material likely has been discarded. We may never know exactly how cross contamination occurred," he said.

Skinner said outside investigators from the USDA's Animal and Plant Health Inspection Service (APHIS) concluded its investigation into the bird flu mishap on Friday.

Frieden has pledged to make sweeping changes to improve safety measures at CDC labs handling dangerous bacteria and viruses. It has shut down the two labs involved in the anthrax and bird flu incidents and has suspended the transfer of samples from high-security labs until their safety procedures are reviewed.

The agency is also assembling a group of outside experts to advise on biosafety. That panel could be announced later this week, Skinner said.

(Reporting by Julie Steenhuysen; Editin