The Melbourne based drug manufacturer CSL has been accused by the US Food and Drug Administration (FDA) for not doing enough to investigate why its flu vaccine caused fever and convulsions in children.

The FDA has issued a warning to CSL after it failed to provide an adequate response to concerns about its flu vaccine and its manufacturing practices.

The flu vaccine, Fluvax or Afluria as it was marketed in the US had caused hundreds of Australian children under five to develop convulsions and fevers. CSL withdrew the vaccine from the market in June last year.

CSL has yet to discover the reason for the serious reaction to the vaccine prompting the FDA to call the investigation not good enough. The watchdog sent inspectors to CSL's headquarters last March. The FDA told CSL it was not happy with what it found, according to a report from ABC News.

"Failure to promptly correct these deviations may result in regulatory action without further notice," the FDA letter dated June 15 said.

A CSL spokeswoman said the company is now working to submit a detailed response to the flu vaccine debacle. The FDA set a 15 day deadline for the company and will meet with company officials in the first half of the next month.

''If they say our response is adequate we'll continue to implement our action plan,'' Ms Sharon McHale, a CSL spokeswoman said.

''If they feel as though we haven't complied, then they can take regulatory action.''

Children who had taken the CSL vaccine were between 3 to 6.5 times more likely to have a fever than other children who were vaccinated with other products.

The FDA's warning letter has some serious repercussions for Australia's biggest biotechnology company. If CSL can't comply with the FDA then it wouldn't be able to market its product in the US which would also negatively impact its Australian market share.

Fluvax is no longer sold for use on children under five and it is only recommended for children aged five to ten if there are no other vaccines available.