COLOMBO, Sri Lanka — Sri Lankan customs agents have seized 550 pounds of heroin worth $19 million from a container sent from Pakistan, in the largest seizure of the drug in South Asia, an official said Saturday.

The heroin, concealed inside 17,500 small grease cans packed into a large container sent from Karachi, was found Friday at a customs terminal in Sri Lanka’s capital, Colombo, said Mali Piyasena, a customs director. One Pakistani and three Sri Lankans were arrested in connection to the seizure, Mr. Piyasena said.

He said customs agents had received a tipoff about the heroin in July and were “waiting till the importer came to clear it.”

A police official involved in the investigation said that the authorities had known of a Pakistani narcotics ring that supplies heroin for Sri Lankan drug dealers and for shipment to Europe. The official, who spoke on the condition of anonymity because he was not authorized to speak to the news media, said the authorities received information that part of this stock was for Sri Lanka, and the rest was for Europe.