A US cleaning service that hired more than 100 children at 13 meat-packing plants has been fined more than $US1.5 million ($2.1 million). 

US Department of Labour officials found 31 underage workers as young as 13 working for Packers Sanitation Services Inc (PSSI) last year at meat-packing plants owned by JBS USA and Turkey Valley Farms.

They also searched PSSI's headquarters where they found more underage workers.

The department went on to review records for 55 locations where PSSI provided cleaning services and found even more violations, involving children aged 13 to 17.

The agency obtained a temporary restraining order in November and a permanent injunction in December, when PSSI entered into a consent judgement that committed the company to no longer employ minors illegally.

Over the past three years, children were found to be using caustic cleaning chemicals and cleaning "dangerous power-driven equipment like skull-splitters and razor-sharp bone saws," Jessica Looman from the department said.

At least three of those minors, including a 13-year-old, suffered burns from the chemicals used for cleaning at the JBS plant in Nebraska, officials said.

Some of the children worked overnight shifts and were also enrolled in schools during the day, department spokeswoman Rhonda Burke said.

The fine PSSI paid was $US15,138 for each minor, which is the maximum allowed under federal law.

But investigators believe the company employed many more than the 102 children.

Under the consent judgement, Ms Looman said PSSI must identify and remove them from dangerous work.

"These findings represent a systemic failure across PSSI's entire organisation. 

"PSSI's systems in many cases flagged that these children were too young to work, and yet they were still employed at these facilities."

The company's vice-president of marketing, Gina Swenson, said the company had "a zero-tolerance policy against employing anyone under the age of 18". 

She said as soon as PSSI became aware of the allegations it conducted audits and hired an outside law firm to help strengthen its policies.

PSSI has also conducted additional training for hiring managers, including on spotting identity theft, she said.

None of the minors identified by federal investigators still work for PSSI, and the Department of Labour has "not identified any managers aware of improper conduct that are currently employed", Ms Swenson added.

The United Food and Commercial Workers International Union, which represents meat-packing plant workers, called PSSI "one of the worst actors" in the industry.

"Paying a simple fine is not enough, their entire business model relies on the exploitation of workers, vicious union-busting tactics, and the violation of human rights," Marc Perrone, the union's international president, said.