More than 21,000 'yaba' tablets worth over $540,000 were seized at the Woodlands Checkpoint on Tuesday evening, in the largest single seizure of such tablets since 1999.

A total of four suspects have been arrested in connection with the case, said the Central Narcotics Bureau (CNB) and the Immigration and Checkpoints Authority (ICA) in a joint statement issued on Wednesday.

At about 7.45pm on Tuesday, ICA officers inspected a Thai-registered car driven by a 51-year-old male Thai national. He was immediately arrested after four black packages were found hidden behind the car's glove compartment. These four packages had a total of 21,910 'yaba' tablets - also known as methamphetamine. If convicted, he may face the death penalty.

CNB launched two follow-up operations, which led to the arrest of two Malaysian men and one Singaporean woman aged between 35 to 41 in the Jalan Sultan area. They are suspected to be connected to the Thai national and the drug consignment.

Investigations into the other possible drug trafficking activities of the trio are ongoing.

CNB director Ng Ser Song said: "Together with our Home Team partners, CNB will maintain our relentless enforcement at the checkpoints to prevent individuals or drug syndicates from smuggling in illicit drugs."