Federal authorities have seized unauthorized versions of the Covid-19 treatment remdesivir at US airport, which was destined for distribution in Mexico, the Wall Street Journal reported on Wednesday.

Versions of antiviral remdesivir, without licenses from the drug's manufacturer Gilead Sciences Inc, were arriving in the United States by plane from Bangladesh and India, the newspaper said.

They were being smuggled by individuals to Mexico for patients willing to pay top dollar for the drugs, the report added, citing people familiar with the investigation.

"We caution against sourcing Gilead medicine from outside the approved and regulated supply chain, hospitals or pharmacies and will continue to support US law enforcement in taking appropriate actions to protect patients," Gilead said in an email to Reuters.

In recent months, US Customs and Border Protection officers have captured more than 100 shipments that they referred to US Department of Homeland Security special agents for further investigation, according to the report.