The Central Narcotics Bureau on Thursday seized 4kg of heroin worth more than $428,000, in the largest haul of the drug so far this year.

Two men - a 23-year-old Malaysian and a 36-year-old Singaporean - were arrested by officers who mounted an operation in Woodlands.

Just after 7am on Thursday morning, the two men were observed meeting up at a multi-storey carpark at Woodlands Drive 50.

They were arrested in the midst of their meeting.

From the rear storage box of the motorcycle belonging to the Singaporean man, officers recovered about 2.7kg of heroin, wrapped in six different bundles.

A black bag that the Malaysian man was carrying, contained about 1.3kg of heroin wrapped in three different bundles.

Both men are being investigated for drug trafficking. If convicted, they may face the death penalty.