Two foreign trawlers packed with over $33m worth of heroin and crystal methamphetamine were seized in Sri Lanka's biggest-ever drugs bust at sea, a navy spokesman said on Thursday.

Troops found 400kg (about 880 pounds) of heroin and 100kg (about 220 pounds) of the drug "ice" on board the two vessels, spokesman Isuru Sooriyabandara said.

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The street value of the seized narcotics was estimated by officials to be six billion rupees ($33.5m).

The vessels, as well as a Sri Lankan fishing boat, were escorted to a harbour near the capital Colombo on Thursday.

Sixteen foreign crew and five Sri Lankan crewmen were detained for questioning, Sooriyabandara added. The nationalities of the foreigners were not disclosed by authorities.

"The two foreign vessels were trying to offload the drugs onto a Sri Lankan fishing boat," Sooriyabandara told the AFP news agency.

The foreign vessels, which carried no registration, were intercepted in international waters some 1,100km (684 miles) from Sri Lanka's coast.

Sooriyabandara said investigators were checking GPS data on board the vessels to establish the route and point of origin.

Authorities said initial investigations suggested the Iran-Pakistan coast was the point of origin.

In 2016, the Sri Lankan navy arrested 10 Iranians, two Pakistanis, one Indian and one Singaporean national for allegedly trying to smuggle 110kg (243 pounds) of heroin into the island nation via a trawler.

The country's previous largest drug haul was in December 2016 when police seized 800kg (1,764 pounds) of cocaine on a ship at Colombo's port from Saudi Arabia's King Abdullah Port.

Six months earlier, authorities discovered 301kg of cocaine inside a shipping container at Colombo.

Authorities believe the Indian Ocean island is also being used as a trafficking transit point.