Sales of medically important antibiotics in the United States for use in livestock jumped by 20 percent between 2009 and 2013, federal regulators reported on Friday, data that is sure to fuel concern that bacteria infecting humans could grow more resistant to the drugs.

The U.S. Food and Drug Administration reported that domestic sales and distribution for such drugs approved for use in cattle, chickens, hogs and other food animals increased 3 percent between 2012 and 2013, according to the annual report.

But the current sales picture of such drugs could be different.

Some leading U.S. chicken producers, including Tyson Foods and Perdue Farms, have said in the past year they have either reduced or eliminated the use in barns and poultry hatcheries of antibiotics used on humans.

"A lot of these announcements came in either late 2013 or in 2014, so we don't really expect to see that reflected in the data we're looking at right now," said Gail Hansen, a senior officer for Pew Charitable Trusts' antibiotic resistance project. "At some point, though, we should be seeing a decrease."

Public health advocates, along with some lawmakers and scientists, have criticized the long-standing practice of using antibiotics in livestock, saying it is fueling the rise of antibiotic-resistant bacteria.

Agribusinesses defend the practice, saying animal drugs are needed to help keep cattle, pigs and chickens healthy, and increase production of meat for U.S. consumers.

What specific antibiotics are being fed to which animals, and in what volume and for what reasons, is not clear. So even though the FDA sales data is more than a year old, Hansen said, it will help federal regulators create a baseline for their current research efforts.

The U.S. Department of Agriculture plans to begin collecting more detailed data on antibiotics used on farms in a potential precursor to reducing use of such drugs in livestock.

The agency is awaiting funding approval for the research.

Last month, the White House issued a sweeping plan to slow antibiotic resistance over the next five years. While the Obama Administration's plan set clear goals for reducing such infections in humans, it did not specify limits for the agricultural sector.

The FDA's summary report on antimicrobial drug sales for use in livestock can be found here: here