US health authorities pressed farmers to give fewer antibiotics to livestock and poultry to reduce the risk of potentially harmful resistance to antimicrobial drugs.

Animals developing resistance to the drugs and the antibiotics' resulting loss of effectiveness "poses a serious public health threat," the US Food and Drug Administration warned as it issued guidance on the issue.

Yet, FDA Centre for Veterinary Medicine director Bernadette Dunham stressed the drugs could play a key role - when used properly.

"Using medically important antimicrobial drugs as judiciously as possible is key to minimising resistance development and preserving the effectiveness of these drugs as therapies for humans and animals," she said in a statement.

The FDA recommended that farmers limit their use of "medically important" antimicrobial drugs in food-producing animals seen as necessary to ensure the animal's health so long as a veterinarian oversees or consults on the process.

According to the draft guidance, which summarises a number of related published reports, most evidence supports the conclusion that using the drugs for non-therapeutic uses, such as to enhance production and growth "is not in the interest of protecting and promoting the public health."

But National Pork Producers Council president Sam Carney rejected the FDA's reasoning, which he said "could eliminate certain antibiotics that are extremely important to the health of animals."

The "FDA didn't present any science on which to base this, yet it could have a tremendous negative impact on animal health and, ultimately, the safety of food," added Mr Carney, a pork producer from Adair, Iowa.

"As we know, healthy animals produce safe food, and we need every available tool to protect animal health."