Philippine authorities say they have seized more than half a ton of suspected methamphetamine in suburban Manila and arrested six people in what could be one of the largest drug seizures under President Rodrigo Duterte.

Police say National Bureau of Investigation agents initially seized about 100 kilograms (220 pounds) of the substance, locally known as shabu, in six bags and arrested four Filipinos in a car in the upscale Greenhills residential district Friday.

Almost simultaneously, other agents raided a nearby house and found about 500 kilograms (1,100 pounds) of meth and arrested two suspects believed to be Chinese.

Duterte has been waging a brutal crackdown against illegal drugs since he took office in June. More than 6,000 suspected drug users and dealers have been killed, alarming human rights advocates