Vietnamese police say they have made the largest drug haul in the country's history, seizing nearly nine tons of cannabis resin in cartons which were en route from Pakistan to China.

The drugs were found in 400 cartons covered by jeans, and are estimated to have a street value of nearly $US90 million.

Anti-narcotics police officers found the drugs in a warehouse in Mong Cai town on the border with China.

Five people carrying Indonesian or Chinese passports have been detained on suspicion of owning two containers being used to transport the drugs.

"This is the biggest volume of drugs transported into Vietnam by sea," the Anti-Drug National Committee said.

Despite Vietnam's strict drug-trafficking laws, which include the death penalty for some offences, it continues to be used as a transit point for heroin, opium, amphetamine pills and other illegal drugs.

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