South Korea discovered and seized two tons of cocaine hidden aboard a Norwegian-flagged vessel, authorities said on Friday, marking the country's largest drug bust to date.

The Korea Coast Guard said they had found "two tons" of what they suspect to be pure cocaine on a Norwegian-flagged vessel which had departed from Mexico and made stops in Ecuador, Panama, and China.

The operation was launched after South Korean authorities received intelligence from US agencies – the FBI and Homeland Security Investigations (HSI) – that the vessel was carrying hidden narcotics.

The Korea Coast Guard and Korea Customs Service coordinated a large-scale search operation "consisting of a joint search team of 90 officers... along with two drug-sniffing dog units," the Korea Coast Guard said.

After the vessel docked at an east coast port in South Korea, the team immediately boarded the vessel and "discovered a hidden compartment behind the ship's engine room."

The drugs were packed in 56 sacks, each holding some 30 to 40 kilogrammes of the drug, authorities said, bringing the total haul to around two tons.

"Preliminary field tests confirmed the substances as suspected cocaine," the Korea Coast Guard official said.

"The seizure is the largest in history, about five times bigger than the previous record, which was 404 kilogrammes of methamphetamine," said a Korea Customs Service official.

The estimated street value is one trillion won (US$697 million), he added.

Authorities have launched a joint investigation team to question the vessel's captain and crew about the origin and intended destination of the drugs, and the route used to transport them.

Investigators also said they are looking into potential ties to international drug trafficking syndicates and will expand cooperation with the FBI and HSI.

South Korea has long ranked among the countries with low drug use globally, thanks to its strict laws and strong social stigma. (AFP)