Rakop Srisuppa-art, head of the southern customs unit in Songkhla, said officials had stepped up monitoring for smuggling at the Sadao and Padang Besar checkpoints.

From last October to March, officials at the Padang Besar checkpoint arrested smugglers with a total of 60,305 litres of petrol. During the same period, another 4,695 litres of smuggled petrol were also confiscated.

In the first half of this month, officials confiscated 3,100 litres of smuggled petrol at Padang Besar checkpoint.

The domestic price of diesel has topped a record Bt26 per litre while petrol is now over Bt27 per litre. Retail petrol prices in Malaysia are significantly lower.

Rakop said smuggling via the Sadao checkpoint had fallen because Malaysian officials were strictly enforcing rules and regulations to prevent smuggling.

Thai motorists are allowed to buy up to 20 ringgit (Bt207) worth of diesel and can fill up their tanks with petrol.

Thai customs officials earlier discovered that some smugglers had redesigned their vehicles' tanks in order to store more petrol for resale in Thailand.

Meanwhile Pramote Kongthong, president of a truck-operators' association in the Northeast, said rising diesel prices had significantly increased the operating costs of transport companies so they had sought a 4-per-cent increase in rates.

A shortage of diesel in Ubon Ratchathani, Si Sa Ket, Surin and Mukdahan is making it difficult to deliver goods in the provinces, he said.

"Each of the large delivery trucks needs a 200-litre full tank, but the drivers cannot fill up their tanks now when they arrive at these destinations. Some even have to hire smaller vehicles to buy diesel from other places so that the large trucks can make the return trips," Pramote said.