Washington (AFP) - US authorities warned Monday that "mass quantities" of counterfeit prescription drugs laced with the opioid fentanyl are being produced in Mexico for distribution in North America.

A sample of tablets seized in the US found that 27 percent contained potentially lethal doses of fentanyl, according to the Drug Enforcement Administration.

Fentanyl is a synthetic opioid that US authorities blame for more than 100 deaths a day in the United States.

Mexican drug cartels "are now sending counterfeit pills made with fentanyl in bulk to the United States for distribution," said the DEA's acting chief, Uttam Dhillon.

"Counterfeit pills that contain fentanyl and fentanyl-laced heroin are responsible for thousands of opioid-related deaths in the United States each year," he said.

On average, 130 people a day die in the United States of opioid overdoses, according to government statistics, which show that fentanyl is involved in more deaths than any other drug.

The drug is 50 times more potent than heroine, with only a few milligrams enough to cause death.

The DEA considers a lethal dose to be about two milligrams, but that varies according to an individual's physical size, tolerance, previous consumption and other factors.

China, the biggest supplier of fentanyl in the United States, in May banned all precursor chemicals of fentanyl, an action applauded by the administration of Donald Trump.

But experts at the time said the Chinese action could incentive production of those chemicals elsewhere, including Mexico.