The International Labour Rights Fund has filed a federal lawsuit in the United States alleging workers at Bridgestone Corp's Firestone rubber plantation in Liberia toil in virtual slavery.

Firestone, which has operated a rubber plantation in Liberia since 1926, relies on a poverty stricken and often illiterate work force to tap tons of raw latex from rubber trees using primitive tools and methods that expose them to dangerous pesticides and fertilisers, the group said.

Dan Adomitis, the head of Firestone Natural Rubber Co, the unit that operates the Liberian operation, said the tyre maker will defend itself.

"This group has a long track record of filing and losing similar lawsuits against corporations," Adomitis said in a statement, referring to the labour rights group.

"The allegations by this group are outrageous and are simply not supported by the facts. This has more do with generating headlines than with seeking justice."

"Firestone is doing more than any other private entity to invest in the rebuilding of Liberia in the aftermath of a decades-long war. We provide our (employees) in Liberia with stable jobs, housing, health care and free education," added Adomitis, who was named in the lawsuit.

He said Firestone's jobs are among the highest paying in Liberia, the employees are represented by a union and child labour is prohibited.

The International Labor Rights Fund in September sued retailer Wal-Mart Stores Inc, alleging it has overlooked sweatshop conditions at factories in six countries.

Firestone's Liberian workers cannot meet daily harvesting quotas without unpaid aid, requiring them to put their own children to work or face starvation, the fund said in the lawsuit, which seeks class action status and was filed in federal court in California.

Practices differ little from when the plantation opened, with Firestone relying on forced labor, involuntary servitude, recklessness, negligence in hiring and supervision, unjust enrichment and unfair business practices, the group contends.

"The plantation workers are stripped of rights, they are isolated, they are at the mercy of Firestone for everything from food to health care to education, they risk expulsion and certain starvation if they raise even minor complaints, and the company makes willful use of this situation to exploit these workers as they have since 1926," the lawsuit contends.

The lawsuit, filed on behalf of workers and their children at the plantation under pseudonyms, names Japanese parent company Bridgestone Corp, Bridgestone Americas Holding Inc, Bridgestone Firestone North American Tyre and other units as defendants.

The action was brought in the United States because, according to the US State Department, the judicial system in Liberia is "largely dysfunctional" and suffering from the effects of a civil war, according to the lawsuit.

The plantation has an official work force of 6,000 and at least as many child labourers, the group contends, numbering a potential class of up to 4,000 official plantation workers and 10,000 child labourers.

Each official worker must deliver 450 pounds (204 kilograms) of latex per day to meet quotas, requiring them to work up to 14 hours per day and enlist others to help, the group said.