Indonesian authorities early Tuesday seized 1.6 tons of crystal methamphetamine hidden on a Singapore-flagged ship in their second major drug bust this month, officials said.
Customs inspectors said they spotted the vessel between Indonesia's Sumatra island and Singapore and reported it to police.
A subsequent search of the ship turned up the huge haul of narcotics stuffed into some 81 rice sacks. Four Taiwanese crew were arrested including a 69-year-old man.
"We are currently weighing the drugs and questioning four Taiwanese crew," tax and customs agency spokesman Deni Sirjantoro told AFP.
Indonesian authorities said they had been looking for the ship for several months on suspicion it was shipping drugs to Indonesia and Australia, adding that it may have flown flags from different countries to avoid detection.
Earlier this month the Indonesian navy seized 1.3 tonnes of crystal methamphetamine and arrested four Taiwanese crew on a ship spotted in the strait between Singapore and Indonesia.
Indonesia has some of the world's toughest anti-drugs laws, including capital punishment for some trafficking cases.