About 600 kg of illegal stimulant drugs with a street value of some ¥42 billion ($387 million) have been confiscated from a yacht moored at Naha port in Okinawa, investigative sources said Thursday.

The amount is believed to be the largest ever for stimulants confiscated in Japan.

In connection with the case, the Japan Coast Guard served fresh arrest warrants Thursday on six people from Taiwan for suspected violation of the Stimulants Control Law concerning psychoactive agents such as amphetamine and methamphetamine.

The six were initially arrested on suspicion of possessing ketamine in violation of the narcotics control law, when they went through customs after arriving in Naha via Ishigaki Island in the prefecture from Taiwan in May.