More than 100 customs officers are checking trucks for illegal fuel at 19 inspection sites, including three border checkpoints, in their largest crackdown to date. Twelve drivers had been arrested up until last night. The officers have inspected more than 3,500 vehicles since Monday. Two dump trucks have been impounded and 660 litres of illicit diesel worth $2,780 seized. Customs officers said one of the dump trucks' fuel tanks had been modified to allow it to carry illicit fuel oil. The 19 inspections stands have been erected at landfills, mud-dumping sites and car parks. Vehicles being targeted include dump trucks and garbage trucks. Lam Sze-hau, commander of customs' diesel oil enforcement division, said the operation would continue and officers would review the situation next week before deciding how long it would go on. 'The result of the operation, so far, has shown that the number of drivers allegedly using illegal fuel is low,' he said. The operation also aimed to warn drivers to stay away from illicit fuel, he said. It was mounted after officers received complaints from transport unions. 'We were told that a number of people from the transport industry used illicit fuel to cut their operational costs so they could make cheap offers to their clients and get more business,' Mr Lam said. 'But the operation shows that the problem is not serious.' In the first five months of the year, 117 people were arrested for using illegal fuel and four vehicles impounded. Last year, officers arrested 264 people for similar offences and impounded 10 vehicles. Illegal diesel is often made with low-grade oils and other fuels, including kerosene and even vegetable oil. The resulting mixture can contain dangerously high levels of contaminants such as sulfur.