The United States' top public health agency revealed Friday a series of alarming incidents in which dangerous biological agents including anthrax, influenza and botulism were mishandled over the past decade.

The latest revelation involved a mistaken contamination of a mild flu strain with a deadly type of H5N1 bird flu, which was then shipped from the Centers for Disease Control and Prevention (CDC) in Georgia to a separate government lab, authorities said.

The flu incident came in the same week as the discovery of forgotten smallpox vials at a separate US government lab near the US capital, and followed admissions of mishandled anthrax at a CDC lab in Atlanta last month.

Though no one was taken ill by the events, they have raised new concerns about the safety of dangerous agents which could be used as bioterror weapons.

In addition to the three incidents so far this year, the CDC acknowledged three more in the past decade in which biological materials were shipped without being properly inactivated, including anthrax and Clostridium botulism in 2006, and in 2009 a contagious strain of Brucella, which causes an infectious disease known as brucellosis.

CDC director Tom Frieden said he was "astonished" that protocols could have been violated in that way, and described the latest flu incident as the "most distressing."

The CDC said the cross-contamination was unintentional, but the lab that shipped it is closed until better safety measures can be put in place.

"Everything we have looked at strongly suggests that there was no exposure of anyone to influenza," said Frieden at a press briefing.

He told reporters he had lost sleep since learning of the flu mix-up on Wednesday, six weeks after it occurred on May 23.

"These events should never have happened," he said.

"I am disappointed and frankly, I'm angry about it."

- Moratorium -

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 who knowingly violated protocol or failed to report a lab incident.

"It tells me we need to look at our culture of safety throughout all our laboratories," Frieden said.

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

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

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

Frieden said the type of H5N1 shipped was not the exact same as the one that spread in China, but "it is one of the ones that does concern us because it can be quite deadly both for poultry and for people."

The other type of flu involved was H9N2, he said.

The incidents will raise further concerns among opponents of studies known as "gain of function," in which scientists manipulate flu strains to find out how they can more easily spread, as well as how to better vaccinate against them.

In 2011 and 2012, worldwide scientists self-imposed a moratorium on such research after concerns were raised about research that made a potent flu strain jump easily between mammals.

The full details of those experiments were eventually published in leading scientific journals.

- 'More control' -

"It's disturbing. It's a little scary," said Len Horovitz, a pulmonary specialist and internist at Lenox Hill Hospital in New York, regarding the latest CDC revelations.

"The CDC has to have more control of its labs. More oversight is needed."

The CDC has also completed initial testing on the six apparently forgotten vials of smallpox that were found in a Food and Drug Administration lab at the National Institutes of Health earlier this week, and two had viable smallpox in them, Frieden said.

After scientists complete their tests, they will destroy the samples in view of World Health Organization officials, he said.

"That is what should have been done a couple of decades ago," Frieden admitted.

"Whoever created these vials did not do so out of malice," he said, adding that they were dated February 10, 1954.

Smallpox was not eradicated worldwide until the late 1970s.