BANGKOK (AFP) - Thai police on Friday (Dec 16) unveiled a record half tonne seizure of crystal methamphetamine, as authorities target a key overland drug route through the kingdom and into Malaysia.

The haul of high-grade meth, better known as ice, has a street value of around US$40 million, the Narcotics Suppression Bureau (NSB) said in a statement.

Seven Thais have been charged with drug trafficking, it added.

Wednesday night's bust unfolded after a car stopped suspiciously short of a police patrol in southern Chumphon province.

A police search found nothing in the vehicle.

"But the driver was acting suspiciously, refusing to answer mobile phone calls in presence of police... he began to shake and sweat," the NSB said.

Officers scoured the road for other suspicious cars, aware that drug runners often operate in convoys, and found two suspects in another vehicle who confessed to acting as lookouts for a truck carrying the meth.

The 18-wheel vehicle was found parked at a petrol station in Chumphon town with 500 bars of the drug wrapped in black plastic bags and hidden under sacks of corn husks, the NSB said.

"It's a record seizure of these drugs (ice)," Major General Dusadee Choosangkij told AFP.

"I believe they were to be stored in Malaysia and then destined for either Australia or Taiwan."

Ice, which is highly addictive, sells for around three million baht (around US$80,000) a kilogramme on Bangkok's streets.

With long land borders, Thailand is a key route for ice, "yaba" - the pill version of meth - and heroin produced in factories in Myanmar and Laos.

Much of the production is controlled by Myanmar's powerful and heavily armed Wa ethnic group, whose high quality pills and heroin are stamped with their own logos.

The drugs are transported by car or foot across remote borders and sold to Thai gangsters.

Last week six Wa tribesmen were killed in a late night shoot-out with a Thai army patrol near the rugged Golden Triangle border region.