The Narcotics Suppression Bureau admitted on Sunday that at least 11 drug manufacturing facilities were located "near Thailand's northern border" with a combined daily production capacity of more than 20 million pills.

The authority is wary of a possible influx of illicit drugs into the country during the Songkran festival this week.

Pol Lt Gen Sommai Kongwisaisuk, the bureau commissioner, said the agency was keeping a close watch on the situation. He did not name the country where the drugs are made and smuggled, which is Myanmar.

Details of the drug plants first emerged on the CSI LA Facebook page. It claimed the plants were preparing to flood Thailand with illicit drugs for Songkran.

CSI LA is best known for exposing the luxury watch scandal involving Gen Prawit Wongsuwon. The Facebook page believes that the government is withholding information on the drug situation.

It claimed that during the past three months, over two billion speed pills and more than 10 tonnes of ice (crystal methamphetamine) was smuggled into Thailand, but only one-third of that amount was seized.

Pol Lt Gen Sommai said authorities were working with Myanmanr to combat the illicit drugs trade.

The Tourist Police Bureau said it was stepping up a campaign to stem the flow of narcotics into the kingdom during Songkran, when 900,000 foreign visitors are expected.

Pol Maj Gen Surachet Hakpal, deputy commissioner, said the bureau was working closely with other agencies to monitor tourist groups suspected of being hired to smuggle drugs.

Meanwhile, three men were arrested and a total of 28 kilogrammes of heroin, with a street value of 100 million baht, were seized on Saturday at Bangkok's Khlong Toey port.

Kulit Sombatsiri, the department's director-general, said on Sunday that 80 bars of heroin were found hidden in 40 rubber pillows declared by Y&S Solutions Trading Co and bound for Hong Kong.

Authorities became suspicious when images from an X-ray scanner at the port showed rectangular objects inside the pillows.

Three men, one Thai and two from Hong Kong, were detained and handed over to the police Narcotics Suppression Division for questioning. The company will also face a charge of attempting to smuggle heroin out of the country.

Mr Kulit said the company, which was registered in January this year, would be blacklisted by the Customs Department after the drug haul. Since January this year, the department seized large amounts of illicit drugs worth about 144 million baht.